An object of the present invention is to provide an outdrive unit that allows space to be saved in the positioning of a hydraulic circuit for trolling, and enables easy maintenance of the hydraulic circuit for trolling. The invention provides an outdrive unit mounted outside of a boat, including: a propeller for propulsion; a hydraulic clutch that controls driving of the propeller; a transmission mechanism that transmits drive power from the hydraulic clutch to the propeller; a hydraulic circuit that operates the hydraulic clutch, the hydraulic circuit comprising a switching valve that switches a rotational direction of the propeller, a pressure-reducing valve for trolling that reduces a pressure of a pressurized oil supplied to the hydraulic clutch, a filter for the pressurized oil introduced to the pressure-reducing valve, and a hydraulic pump that supplies the pressurized oil to the hydraulic clutch; a housing that houses at least one of the hydraulic clutch, transmission mechanism, and hydraulic circuit; a base on which the switching valve, pressure-reducing valve, and filter are mounted, the base being detachably mounted in the housing.