Kaga wrote several novels based on his time in France, including Arechi o tabi suru monotachi (Travelers through the Wasteland) and Furandoru no fuyu (Winter in Flanders) which won the Minister of Education Award for New Artists in 1968.

His 1973 novel Kaerazaru natsu (帰らざる夏, A Summer Long Gone), on the tragic consequences of a young man's military indoctrination during World War II received the Tanizaki Prize.

His 1982 historical fiction about World War II, Ikari no nai fune (Riding the East Wind), has been translated to English to good reviews.

In 1987 he converted to Catholicism at the age of 58 through the influence of Shusaku Endo.

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