We propose a construction of an organic light-emitting field-effect transistor for producing spectrally-narrowed emissions (SNEs) under current injection. The device is characterized by the followings: a diffraction grating fabricated within the channel region, homogeneous source and drain electrodes made of a low work-function metal and a layered structure of organic semiconductor materials. The organic layered structure was composed of a p-type crystal and an n-type thin film. The latter film was deposited so as to cover the grating on the channel. We observed SNEs from the device when it was operated by applying square-wave alternating gate voltages. The observed spectra were peaking as a dominant line around 578nm with a full width at half maximum less than 4nm.