An automatic transmission has a gear transmission mechanism with a a forward clutch which is engaged when the gear transmission mechanism is engaged to a forward speed stage, a one way clutch which is in cooperation with the forward clutch provides a first speed stage by rotation prevention, and a brake which, when engaged, in cooperation with the one way clutch prevents the power output shaft of the gear transmission mechanism from rotating in the direction to allow reverse motion of the vehicle. An idling control system for the transmission includes a first mechanism for engaging the forward clutch selectively either to a maximum amount or to an intermediate amount; a second means for selectively engaging the brake; and a third mechanism for controlling the first and second mechanism. When the transmission is in a forward driving mode, the vehicle is substantially stationary, the engine is substantially idling, and the braking system is applied, the third mechanism engages the forward clutch to the intermediate amount and engages the brake. Thereafter, if the application of the braking system is terminated, it engages the forward clutch to the maximum amount and continues engaging the brake, or, if the engine ceases to be in substantially idling condition, it engages the forward clutch to the maximum amount and disengages the brake. Thereby, rolling of the vehicle backwards down a hill while waiting in the stopped condition with the engine idling is positively prevented, and mild traffic creep is also provided.