A mode transition control device for a hybrid vehicle, the hybrid vehicle having an internal combustion engine configured to be mechanically coupled to a drive wheel via a first engagement clutch that is engaged by an engagement stroke from a released position, a first electric motor configured to be mechanically coupled to the drive wheel via a third engagement clutch that is engaged by an engagement stroke from a released position, a second electric motor that is mechanically coupled to the internal combustion engine, and a battery that is electrically coupled to the first electric motor and the second electric motor, the mode transition control device comprising a mode transition controller configured to carry out a control such that the first engagement clutch is switched from a released state to an engaged state, power generation by the second electric motor is stopped, and a traveling mode is caused to transition to a parallel HEV mode, in which the first electric motor and the internal combustion engine are used as the drive sources, characterized in that: the mode transition controller is configured to carry out a control such that, at the time of start, when a charge capacity of the battery is equal to or less than a predetermined value, the third engagement clutch is switched from a released state to an engaged state in a starting region by not having a slipping element as a starting element and traveling of the hybrid vehicle in a series HEV mode is started, in which the first electric motor is utilized as a drive source and receives electrical power generated by the second electric motor and the battery; the mode transition controller is configured to carry out a control such that, when a vehicle speed becomes a switchover vehicle speed after starting the traveling in the series HEV mode, the traveling mode is caused to transition to the parallel HEV mode; when a temperature rise of a second power generation system including the second electric motor is predicted after starting the traveling in the series HEV mode, the mode transition controller is configured to change the switchover vehicle speed to a second switchover vehicle speed which is slower than a first switchover vehicle speed used prior to a determination of the temperature rise; the second switchover vehicle speed is set to a vehicle speed value that corresponds to an internal combustion engine rotational speed at which the internal combustion engine is capable of self-sustaining operation; and the mode transition controller is configured to set the second switchover vehicle speed to a vehicle speed value that decreases as a road surface gradient decreases, when changing the switchover vehicle speed from the first switchover vehicle speed to the second switchover vehicle speed. an internal combustion engine configured to be mechanically coupled to a drive wheel via a first engagement clutch that is engaged by an engagement stroke from a released position, a first electric motor configured to be mechanically coupled to the drive wheel via a third engagement clutch that is engaged by an engagement stroke from a released position, a second electric motor that is mechanically coupled to the internal combustion engine, and a battery that is electrically coupled to the first electric motor and the second electric motor,