Under the new constitution, the prime minister was no longer selected by the Emperor, but elected by the Diet, "before the conduct of any other business" – and the Socialists pushed for an early vote to prevent the other two major parties from excluding them from a ruling coalition: on May 23, Socialist Tetsu Katayama was elected almost unopposed (420 votes of 426 present in the House of Representatives, 205 of 207 in the House of Councillors) while the coalition negotiations were still in progress.

When Katayama formally became prime minister on May 24 after his ceremonial investiture by the Emperor, he technically held all ministerial posts – a so-called "one-man cabinet" (hitori naikaku, 一人内閣).

Together, the three parties held a solid majority in the HR, and were able to control the HC given the fact that the largest group there, the Ryokufūkai formed by independents, was willing to support the government.

After conflicts over price controls and taxes, the left wing of the Socialist Party threatened to block the budget for fiscal 1948 (begins in April), and in February 1948, Katayama resigned.

Deputy prime minister Hitoshi Ashida was elected on February 21 to succeed him, the Katayama Cabinet remained in office until his investiture on March 10.