The start system related to the present invention is for an internal combustion engine of a hybrid vehicle. The hybrid vehicle is configured to use at least one of a differential mechanism and a transmission gear mechanism to temporarily change an engine speed in starting of the internal combustion engine. The start system includes an electronic control unit. The electronic control unit is configured to determine a duration for which the engine speed needs to be changed in starting of the internal combustion engine. The electronic control unit is configured to determine a degree of change in the engine speed in starting of the internal combustion engine. The electronic control unit is configured to determine whether the engine speed needs to be changed by changing the gear position of the transmission gear mechanism based on the degree and the duration.