Abstract: The main switch is turn on to supply electricity to the control unit for constant-speed crusing control, and a target vehicle speed is set on the basis of an instruction signal from a set switch. The constant-speed cruising control is executed so as for the actual vehicle speed to reach the target vehicle speed. As one of conditions for executing the constant-speed cruising control, the lapse of a predetermined time period from the start of the main switch is added. This arrangement enables the start of execution of the constant-speed cruising control only when the self-diagnosis of the control unit or initialization of various data has been completed.

Abstract: The constant-speed cruising control is performed on the basis of instruction signals from a set switch or the like so that the vehicle speed will reach a target vehicle speed. The constant-speed cruising control is executed so that all switches, such as the set switch or the like, which constitute the instruction signal generating unit, are turned off. This can inhibit the execution of the constant-speed cruising control if the set switch becomes fixed and stays in its ON state.