An engine starting device for controlling an engine start of a vehicle by driving a starter relay of the vehicle with a battery of the vehicle as a power supply, the engine starting device comprising: a control processing unit, and a step-up circuit for stepping up the output of the battery and outputting a drive voltage for driving the control processing unit and the starter relay; wherein the control processing unit has a function of performing an engine start control of applying the drive voltage to the starter relay to activate the starter relay in time of the engine start at which an engine starting condition is satisfied, and controlling a step-up operation of the step-up circuit; and wherein the control processing unit performs a control function on the step-up circuit to: execute the step-up operation of the step-up circuit when the drive voltage becomes lower than a second voltage value higher than a minimum operation voltage of the starter relay, and stop the step-up operation of the step-up circuit when the drive voltage becomes higher than the second voltage value, in a predetermined period necessary to start the engine in time of the engine start, and execute the step-up operation of the step-up circuit when the drive voltage becomes lower than a first voltage value higher than a minimum operation voltage of the control processing unit and lower than the minimum operation voltage of the starter relay, and stop the step-up operation of the step-up circuit when the drive voltage becomes higher than the first voltage value, when not in the predetermined period. a control processing unit, and a step-up circuit for stepping up the output of the battery and outputting a drive voltage for driving the control processing unit and the starter relay; wherein the control processing unit has a function of performing an engine start control of applying the drive voltage to the starter relay to activate the starter relay in time of the engine start at which an engine starting condition is satisfied, and controlling a step-up operation of the step-up circuit; and wherein the control processing unit performs a control function on the step-up circuit to: execute the step-up operation of the step-up circuit when the drive voltage becomes lower than a second voltage value higher than a minimum operation voltage of the starter relay, and stop the step-up operation of the step-up circuit when the drive voltage becomes higher than the second voltage value, in a predetermined period necessary to start the engine in time of the engine start, and execute the step-up operation of the step-up circuit when the drive voltage becomes lower than a first voltage value higher than a minimum operation voltage of the control processing unit and lower than the minimum operation voltage of the starter relay, and stop the step-up operation of the step-up circuit when the drive voltage becomes higher than the first voltage value, when not in the predetermined period. execute the step-up operation of the step-up circuit when the drive voltage becomes lower than a second voltage value higher than a minimum operation voltage of the starter relay, and stop the step-up operation of the step-up circuit when the drive voltage becomes higher than the second voltage value, in a predetermined period necessary to start the engine in time of the engine start, and execute the step-up operation of the step-up circuit when the drive voltage becomes lower than a first voltage value higher than a minimum operation voltage of the control processing unit and lower than the minimum operation voltage of the starter relay, and stop the step-up operation of the step-up circuit when the drive voltage becomes higher than the first voltage value, when not in the predetermined period.