A motor-driven supercharger of the invention is provided with a cooling fluid flow rate adjusting device (a flow rate adjuster (37) and a controller (40)) capable of adjusting a flow rate of a cooling fluid. On the basis of the flow rate adjuster (37) and the controller (40), a flow rate of a cooling fluid (41) is set to a first flow rate (a large flow rate) for cooling an electric motor (20) in a state where a temperature difference obtained by subtracting a temperature of the cooling fluid (41) from a temperature of the electric motor (20) is equal to or more than a predetermined temperature equal to or more than zero, and the flow rate of the cooling fluid (41) is set to a second flow rate (a small flow rate) which is smaller than the first flow rate and is for suppressing a heating effect to the electric motor (20) in a state where the temperature difference is lower than a predetermined temperature equal to or more than zero.