An electronic control unit includes processing circuitry. A hybrid electric vehicle has an electric traveling mode, in which the hybrid electric vehicle travels with a system clutch disengaged and an engine in a stopped state, and a hybrid traveling mode, in which the hybrid electric vehicle travels with the system clutch engaged and engine operating. The processing circuitry is configured to control, when the traveling mode is switched from the electric traveling mode to the hybrid traveling mode, a throttle opening degree of the engine at the time of completion of engagement of the system clutch in accordance with the atmospheric pressure such that a constant intake air amount is obtained regardless of the level of the atmospheric pressure.