There is provided an idling engine speed control apparatus which can economically and suitably control an idling engine speed in the case where a variable displacement type compressor with a variable displacement is mounted is a part of an automotive air conditioning system. A the time of idling, a duty ratio of a control valve current value sent as an external signal to a control valve is sent also to an engine controller, and the engine control apparatus controls the output of an engine on the basis of the duty ratio. For example, when the duty ratio becomes high from a small value, since the present operating state of the compressor is in the direction where the differential pressure becomes high and the displacement is increased, it is expected that a large load is applied to the engine. Thus, in this case, the engine output is controlled correspondingly to a large duty ratio and the idling engine speed is made high.