To provide an engine speed control device which can detect an insufficiency of supplied power from an alternator (generator) during idle operation so as to increase the volume of supplied power from the alternator in order to prevent a battery from running out.

An engine speed control device comprises a voltage regulator 42 for detecting a drop in voltage of a battery 40, and a bypass control valve 10 which is incorporated in an intake-air passage 6 in an engine 1, and which is a solenoid valve of a duty drive type for adjusting an intake air volume fed into a fuel chamber in the engine 1. When a drop in the voltage of the battery is detected, a desired idling speed is reset to a value NTH which is higher than a predetermined value NT, and the drive duty of the bypass control valve 10 is increased so as to increase the intake air volume in order to raise the engine speed during idling operation.