Provided is a motor drive device with which the mounting area of a power circuit can be made large without an electric motor being given a large diameter. A control device (4) comprises: a capacitor assembly (200) provided with a smoothing capacitor (14) for suppressing the ripple component of a current flowing to an electric motor (2); a power circuit (33) that comprises a semiconductor switching element (12a); a heat sink (32) for dissipating heat generated by the power circuit; and a control substrate (20) on which is mounted a microcomputer (18) for generating a drive signal for driving the semiconductor switching element. The heat sink (32) is disposed on the axial line of a rotating shaft of the electric motor (2). The power circuit (33) is mounted on the plane of the heat sink (32) that intersects with the axial line of the rotating shaft (21) of the electric motor (2). The capacitor assembly (200) is disposed on another plane parallel to the plane on which the power circuit (33) is mounted.
