[4]: 44 Matsumoto formed a group called Akamame (赤荳), which is a Japanese translation of the name of a girl in the biographical novel of Modigliani, Les Montparnos, written by Georges Michel.
[5][4]: 45–46 At this time, he became romantically involved with a model, Masayo Iwamoto, but this caused a rift between the friends and the joint atelier was dissolved after five months.
[4]: 83 In the autumn of the same year, Matsumoto went to a museum in Ueno to show Teiko his paintings that had been selected for the Nika Exhibition for the first time.
[4]: 71 Around the same time, his father Katsumi, his elder brother Akira, his wife Teiko, and his mother-in-law Tsune also left Seichō no ie.
In October 1936, he edited and published the first issue of this magazine, Zakkichō (雑記帳; Miscellaneous Notes), financed with the help of his brother Akira.
Writers included Katsuichirō Kamei, Haruo Satō, Shūzō Takiguchi, Sakutarō Hagiwara, Murō Saisei, Tatsuji Miyoshi, and Yojūrō Yasuda, while the members of Ikebukuro Montparnasse as well as painters Katsuzō Satomi, Seiji Tōgō, Tsuguharu Foujita and Sōtarō Yasui contributed articles and illustrations.
[4]: 106 Instead, he earned a living by working as an illustrator for magazines and producing murals for beauty salons and coffee shops run by his friends.
[4]: 126 The issue featured an 11-page roundtable discussion entitled “The National Defence State and Art: What Should Painters Do” (Kokubō kokka to bijutsu: Gaka wa nani o nasubeki ka).
[5]: 551 However, after Asō’s visit, Matsumoto approached the president of Mizue with a request to write a rebuttal, and the magazine agreed to publish a manuscript of 20 pages.
[4]: 3 In the spring of 1943, Matsumoto visited Chōzaburō Inoue, who was living in Ikebukuro Montparnasse, to discuss the formation of a new artist group.
[4]: 139–140 Along with Inoue, Aimitsu, Masao Tsuruoka, Wasaburō Itozono, Gorō Ōno, Masaaki Terada, Saburō Asō, among others, Matsumoto formed the Painting Society of the New Man (Shinjin Gakai; 新人画会).
[5]: 542 However, according to interviews with former members of the Painting Society of the New Man including Asō, Itozono, Inoue, Terada and others, there was no such anti-war statement shared among the group.
[4]: 152 In March 1945, when indiscriminate air raids on the Japanese mainland by the US military intensified, Matsumoto had his wife Teiko, his mother-in-law Tsune, and his eldest son Kan evacuated to Matsue, where his first daughter, Yōko, was born on April 10.
That October (1945), at a time when the debate about war responsibility was raging, Matsumoto submitted an article to the Asahi Newspaper entitled “An artist's conscience” (Geijutsuka no ryōshin).
[4]: 169 Matsumoto declined all of these offers, but he continued to engage professional artists in active discussion independently while pushing forward with a busy exhibition schedule.
In January 1946, Matsumoto sent a pamphlet to many painters containing an article he had written, entitled “Consulting with Artists from Across All of Japan” (Zennihon bijutsuka ni hakaru).
[4]: 170 In November 1946, Matsumoto held a three-person exhibition with Saburō Asō and Yasutake Funakoshi at the Nichidō Gallery in Ginza.
[2]: 369 In February 1948, after the Free Artists' Association exhibition, Matsumoto told his wife Teiko of his intention to move to Paris, a decision that indicates his career-minded aspirations.
[4]: 171 Then, during the three-person exhibition in Gifu in October 1947, his eldest daughter Yōko died of urinary poisoning, and in December Matsumoto himself fell ill with croup after catching a cold.
[2]: 369 [4]: 177 In March, Matsumoto felt strong chest pains, but gave priority to the production of the 2nd Art Group Union Exhibition in May.
[4]: 179 As Matsumoto and Teiko could not afford the cost of hospitalization, he rested at home, but his condition suddenly changed on the morning of June 7 and he died on the following day.