A switch control device for controlling a voltage-controlled type semiconductor switch, which connects a first power supply and a second power supply having a rated voltage lower than that of the first power supply and whose gate is driven by a gate drive voltage supplied from a gate drive circuit, wherein the switch control device is a micro processing unit (MPU, 100) configured to output a control signal for switching ON/OFF of the semiconductor switch to the gate drive circuit and to monitor a potential difference (ΔV), the potential difference (ΔV=V1-V2) being a difference between a voltage of the first power supply and a voltage of the second power supply, wherein the semiconductor switch is set to an ON state in a case where a potential difference (ΔV) is equal to or greater than a positive first predetermined value, wherein a PWM signal is output by the micro processing unit (MPU, 100) to the gate drive circuit to cause the semiconductor switch to be in a continuous or intermittent half-ON state in a case of a first condition that the potential difference (ΔV) is less than the first predetermined value or equal to or less than the first predetermined value and a second condition that the potential difference (ΔV) is greater than a second predetermined value of 0 or less or is equal to or greater than the second predetermined value, and wherein the semiconductor switch is set to an OFF state in a case where the potential difference (ΔV) is equal to or less than the second predetermined value or is less than the second predetermined value, wherein the PWM signal and an ON signal are alternately transmitted to the gate drive circuit so as to alternately switch the semiconductor switch between the half-ON state and the ON state, in the case where the potential difference (ΔV) is less than the first predetermined value or equal to or less than the first predetermined value and the potential difference (ΔV) is greater than the second predetermined value or equal to or greater than the second predetermined value. wherein the switch control device is a micro processing unit (MPU, 100) configured to output a control signal for switching ON/OFF of the semiconductor switch to the gate drive circuit and to monitor a potential difference (ΔV), the potential difference (ΔV=V1-V2) being a difference between a voltage of the first power supply and a voltage of the second power supply, wherein the semiconductor switch is set to an ON state in a case where a potential difference (ΔV) is equal to or greater than a positive first predetermined value, wherein a PWM signal is output by the micro processing unit (MPU, 100) to the gate drive circuit to cause the semiconductor switch to be in a continuous or intermittent half-ON state in a case of a first condition that the potential difference (ΔV) is less than the first predetermined value or equal to or less than the first predetermined value and a second condition that the potential difference (ΔV) is greater than a second predetermined value of 0 or less or is equal to or greater than the second predetermined value, and wherein the semiconductor switch is set to an OFF state in a case where the potential difference (ΔV) is equal to or less than the second predetermined value or is less than the second predetermined value, wherein the PWM signal and an ON signal are alternately transmitted to the gate drive circuit so as to alternately switch the semiconductor switch between the half-ON state and the ON state, in the case where the potential difference (ΔV) is less than the first predetermined value or equal to or less than the first predetermined value and the potential difference (ΔV) is greater than the second predetermined value or equal to or greater than the second predetermined value.