The control technique of the invention sets smaller values to an engine stop reference value and an engine start reference value in the state of use of a regular fuel, compared with values in the ordinal state. When a calculated engine power demand Pe* is less than the engine stop reference value, the control stops the operation of the engine. When the calculated engine power demand Pe* exceeds the engine start reference value, on the other hand, the control starts the operation of the engine. This arrangement facilitates the start of the engine in the state of use of the regular fuel, compared with the start in the ordinary state. A corrective revolution speed N is added to a target revolution speed Ne* of the engine, in order to respond to a decrease in output power from the internal combustion engine.