Provided is a motor drive device that allows for increase in the mounting area of a power circuit without increase in the diameter of an electric motor. A control device 4 includes: a capacitor assembly unit 200 including a smoothing capacitor 14 for suppressing a ripple component of current flowing through the electric motor 2; a power circuit 33 including a semiconductor switching element 12a; a heat sink 32 for dissipating heat generated from the power circuit; and a control board 20 on which a microcomputer 18 for generating a drive signal for driving the semiconductor switching element is mounted. The heat sink 32 is located on the axial line of a rotary shaft of the electric motor 2. The power circuit 33 is mounted on one surface of the heat sink 32 perpendicular to the axial line of the rotary shaft 21 of the electric motor 2. The capacitor assembly unit 200 is located on another surface parallel with the surface on which the power circuit 33 is mounted.
