An idling speed control system for controlling idling rotational speed of an internal combustion engine. A vacuum-operated actuator having a diaphragm defining a vacuum chamber controls opening and closing of a throttle valve in response to first and second control pressures supplied from a change-over control valve to the vacuum chamber. An electronic control unit generates, in response to an on-off control pulse signal having a period corresponding to rotational speed of the engine, one of on-period and off-period of the on-off control pulse signal having a predetermined constant value, and supplies the change-over control valve with the control pulse signal. Preferably, a detector is connected to the electronic control unit for detecting a change in the magnitude of a load on the engine and supplying the electronic control unit with an output signal indicative of the detected change in the load magnitude. The electronic control unit is responsive to the output signal for setting the pulse duration of the on-off control pulse signal to a value different from the above predetermined constant pulse duration. A delay valve provided across a passage connecting between the vacuum chamber and the change-over control valve sets the feeding speed of the first control pressure and that of the second control pressure fed through the passage to different values from each other.