A field-effect transistor, comprising: a semiconductor multilayer structure including a carrier transport layer made of nitride semiconductor; a gate electrode formed on the semiconductor multilayer structure at a position corresponding to a channel region of the carrier transport layer, the gate electrode having a first sidewall surface on a first side thereof and a second sidewall surface on a second side thereof; an insulating film formed directly on the gate electrode to cover at least one of the first sidewall surface and the second sidewall surface; a first ohmic electrode formed on the first side of the gate electrode on the semiconductor multilayer structure; a second ohmic electrode formed on the second side of the gate electrode on the semiconductor multilayer structure; and a passivation film including a first portion extending from the first ohmic electrode toward the gate electrode to cover a surface area of the semiconductor multilayer structure between the structure between the first ohmic electrode and the gate electrode and a second portion extending from the second ohmic electrode toward the gate electrode to cover a surface area of the semiconductor multilayer structure between the second ohmic electrode and the gate electrode, wherein: the insulating film is in direct contact with at least the first and second portions of the passivation film, and has a composition different from that of the passivation film, the passivation film covers said surface areas of the semiconductor multilayer structure except for a gap extending a predetermined distance from the at least one of the sidewall surfaces of the gate electrode, and the insulating film covers at least one of the first sidewall surface and the second sidewall surface and fills the gap, and the insulating film is a single layer or multilayer film, the or each layer of the film comprising an oxide, nitride, or fluoride of a metal element constituting the gate electrode, and the passivation film comprises a silicon nitride film or silicon oxide film. a semiconductor multilayer structure including a carrier transport layer made of nitride semiconductor; a gate electrode formed on the semiconductor multilayer structure at a position corresponding to a channel region of the carrier transport layer, the gate electrode having a first sidewall surface on a first side thereof and a second sidewall surface on a second side thereof; an insulating film formed directly on the gate electrode to cover at least one of the first sidewall surface and the second sidewall surface; a first ohmic electrode formed on the first side of the gate electrode on the semiconductor multilayer structure; a second ohmic electrode formed on the second side of the gate electrode on the semiconductor multilayer structure; and a passivation film including a first portion extending from the first ohmic electrode toward the gate electrode to cover a surface area of the semiconductor multilayer structure between the structure between the first ohmic electrode and the gate electrode and a second portion extending from the second ohmic electrode toward the gate electrode to cover a surface area of the semiconductor multilayer structure between the second ohmic electrode and the gate electrode, wherein: the insulating film is in direct contact with at least the first and second portions of the passivation film, and has a composition different from that of the passivation film, the passivation film covers said surface areas of the semiconductor multilayer structure except for a gap extending a predetermined distance from the at least one of the sidewall surfaces of the gate electrode, and the insulating film covers at least one of the first sidewall surface and the second sidewall surface and fills the gap, and the insulating film is a single layer or multilayer film, the or each layer of the film comprising an oxide, nitride, or fluoride of a metal element constituting the gate electrode, and the passivation film comprises a silicon nitride film or silicon oxide film. the insulating film is in direct contact with at least the first and second portions of the passivation film, and has a composition different from that of the passivation film, the passivation film covers said surface areas of the semiconductor multilayer structure except for a gap extending a predetermined distance from the at least one of the sidewall surfaces of the gate electrode, and the insulating film covers at least one of the first sidewall surface and the second sidewall surface and fills the gap, and the insulating film is a single layer or multilayer film, the or each layer of the film comprising an oxide, nitride, or fluoride of a metal element constituting the gate electrode, and the passivation film comprises a silicon nitride film or silicon oxide film.