A belt tension control device is mounted on a hybrid vehicle that is provided with an engine, a motor, a belt transmitting power between the engine and the motor, and tension changing means capable of changing tension applied to the belt. The belt tension control device is provided with control means for estimating a combustion torque relating to ignition start-up or a motor torque relating to the motor which is required to start the engine before the engine start-up and controlling the tension changing means to change the tension applied to the belt, based on the estimated motor torque before the engine is started, according to the estimated combustion torque so that torque to be transmitted to the engine from the motor is transmitted via the belt.