To provide an engine control circuit not transferring to a low power consumption mode in error even if battery voltage temporarily drops.

A battery voltage detection device 162 set detection signal PD to a low level when voltage of a battery 110 becomes a predetermined value or less. A PD level monitor device 164 measures time when the detection signal PD is kept at the low level, and sets a PD monitor flag inside to 1 when the signal PD is kept at low level foe a predetermined time continuously. CPU 65 inputs the detection signal PD as FIQ signal, checks the PD monitor flag when the FIQ signal becomes the low level and transfers to the low power consumption mode only when value of the PD monitor flag is 1. Since transfer to the low power consumption mode is performed only when low battery voltage time is a predetermined time or longer, transfer to the low power consumption mode is not performed in error even if battery voltage drops temporarily.