To provide a fuel control device for an engine and an idling air/fuel ratio control method, allowing a stable engine speed during idling.

The fuel control device comprises an idling speed control valve 205 bypassing a throttle valve 202 for controlling an engine speed during idling, an air bleed valve 208 installed on the downstream side of a regulator 207 which regulates the pressure of fuel gas for controlling the area of a flow path, idling target speed setting means, throttle valve opening control means for controlling the idling speed control valve, control factor setting means for setting factors to control the opening of the idling speed control valve, trapping means for trapping the state change of the factors, and control means for controlling the air bleed valve 208 in accordance with the trapped state change of the factors.