A field-effect transistor device includes an active area on a semiconductor substrate and a gate electrode, a source electrode, and a drain electrode are disposed on the surface of the active area, so as to define an FET portion. An electrode defining a line for connection to the gate, an electrode defining a line for connection to the source, and an electrode defining a line for connection to the drain are disposed on the semiconductor substrate. The electrodes define a slot line on the input side for supplying a signal to the FET portion, and a slot line on the output side from which a signal of the FET portion is output. The gate electrode has a shape which extends along the direction that approximately perpendicular to the conduction direction of the signal through the slot line on the input side.