An exhaust turbine supercharger, a main engine, and a marine vessel, provided with: a coaxially connected compressor (21) and turbine (22); a motor-generator (32) connected to an axial end of the compressor (21); a space heater (60) that provides heat by supplying current to a winding (66) of a stator (62) in the motor-generator (32); a rotational frequency sensor (54) that detects the rotational frequency of the motor-generator (32); and a control device (5) that stops the space heater (60) when the rotational frequency of the motor-generator (32) as detected by the rotational frequency sensor (54) exceeds a predetermined rotational frequency. Thus, it is possible to suppress dew condensation in the motor-generator, and to operate the motor-generator appropriately.