There is provided an idling rotation speed control apparatus that can reduce the development man-hours and the cost and that enables operation during a trolling cruise to be readily performed. The idling rotation speed control apparatus includes an engine rotation speed detection means, an engine temperature detection means, an engine idling driving state detection means, an engine load detection means, an intake air amount adjusting means for adjusting an amount of intake air during an idling state, and an ECU for controlling the intake air amount adjusting means during idling driving. The ECU includes a basic torque ratio calculation function for calculating a ratio of torque to be generated, to engine maximal torque, that is necessary for making the engine steadily operate at a target rotation speed during idling driving; a target torque ratio calculation function for correcting the basic torque ratio, in accordance with a difference between a target rotation speed and an engine rotation speed, and calculating a target torque ratio; a target air amount calculation function for calculating an air amount necessary for generating the target torque ratio; and an intake air amount adjusting function for controlling the intake air amount adjusting means, based on the calculated air amount.