To provide a drive system for a hybrid vehicle capable of suppressing energy loss by slip control at a start of an internal combustion engine during electric vehicle (EV) travel, a power transmission system for the hybrid vehicle is configured to couple an internal combustion engine, an electric motor, and a transmission by planetary gear mechanisms, has a brake that engages and disengages the internal combustion engine with and from a case, engages the brake during EV travel in which only the electric motor is used as a drive source, and is configured to start the internal combustion engine by disengaging the brake at the time of switching from the EV travel to hybrid vehicle (HEV) travel in which the internal combustion engine and the electric motor are used as drive sources.