In response to simultaneous ON settings of a heater switch signal HSW and an eco switch signal ESW, an eco priority map is set to an intermittent operation permission map (step S350). The heater switch signal HSW is output from a heater switch that is operated to warm up a passenger compartment, while the eco switch signal ESW is output from an eco switch that is operated to give preference to improvement in fuel consumption of the vehicle over the heater function. The eco priority map is designed to allow intermittent operation of an engine in a lower range of cooling water temperature Tw of the engine than that in a heater-on state map set in response to OFF setting of the eco switch signal ESW and ON setting of the heater switch signal HSW. The control procedure of the invention refers to the intermittent operation permission map and determines whether the intermittent operation of the engine is allowed or prohibited, based on the current cooling water temperature Tw and a starting-time cooling water temperature Tst, which represents a temperature of cooling water at a first start of the engine since activation of the vehicle system (step S360).