For a serial multiplex inverter in which each phase includes cells connected serially, wherein each cell includes switching elements and is configured to output a level of +1 (ON), a level of zero (OFF), and a level of −1 (ON) as output levels by operation of the switching elements, a control device includes a switching load distribution control section. This control section is configured to: store information about an ON-output duration of each of the cells and information about an OFF-output duration of each of the cells; for a shift pattern from ON to OFF in the cells, put OFF a gate signal for one of the cells whose ON-output duration is the longest of the cells; and for a shift pattern from OFF to ON in the cells, put ON a gate signal for one of the cells whose OFF-output duration is the longest of the cells.