A key feature of parliamentary democracy is government accountability vis-à-vis the legislature, but the important question of who speaks for the government—cabinet ministers or unelected bureaucrats, and the institutional underpinnings of this behavior—receives scant attention in the existing literature. We investigate this question with the case of Japan, and data on millions of committee speeches spanning distinct electoral and legislative institutional environments. We document how a party-strengthening electoral system reform in 1994 facilitated a dramatic shift in the nature of government accountability to parliament: speeches by ministers increased, speeches by bureaucrats decreased, and discursive accountability between ministers and opposition legislators increased. Subsequent legislative reforms expanding junior ministerial roles and placing explicit limits on bureaucratic participation further reinforced the effects. These findings shed new light on the institutional foundations of responsible party government in general, as well as its progressive development in Japan.