PURPOSE: To make the width size of an installation board short by eliminating a heat-dissipating space between a plurality of inverter devices when the inverter devices are installed side by side on the installation board along the horizontal direction and to easily perform the installation operation of the inverter devices.

CONSTITUTION: Every inverter device is provided with a main circuit part 1 having main-circuit constituent components, with a control part 3 which controls a main circuit and with a cooling fin part 2 to which a semiconductor switching element is attached. A plurality of inverter devices are installed side by side on, and fixed to, an installation board along the horizontal direction. In every inverter device, the control part 3 is arranged on the side of the main circuit part 1, and the cooling fin part 2 is arranged on the back of a device body.