An insulating structure for a power conversion device comprising: a control device of an inverter, the inverter comprising a semiconductor switching device, the semiconductor switching device being connected between two terminals (P, N) having the inverter main circuit voltage, the control device having a control circuit unit configured to generate a first signal; and a gate drive circuit unit configured to receive a gate signal from an optical transmission unit (91, 92) and to control the semiconductor switching device; characterized in further comprising a relay unit provided between the control circuit unit and the gate drive circuit unit, the relay unit having an optical transmission unit (41,44); a signal processing unit; and a power generating unit (43, 45), wherein the optical transmission unit (41, 44) is configured to receive the first signal, to input a corresponding signal into the signal processing unit, to receive a signal from the signal processing unit and to send out the gate signal, and the power generating unit (43, 45) is configured to receive power from a control device AC power source through a first transformer, to supply AC power to the gate drive circuit unit through a second transformer (71, 46, 55), and to supply power to the signal processing unit, and the relay unit, the gate drive circuit unit and the semiconductor switching device are accommodated in an accommodating case that is made of conductive material, and the accommodating case is connected to a middle electric potential terminal having a middle electric potential of said inverter main circuit voltage between said terminals (P, N). a control device of an inverter, the inverter comprising a semiconductor switching device, the semiconductor switching device being connected between two terminals (P, N) having the inverter main circuit voltage, the control device having a control circuit unit configured to generate a first signal; and a gate drive circuit unit configured to receive a gate signal from an optical transmission unit (91, 92) and to control the semiconductor switching device; a control circuit unit configured to generate a first signal; and a gate drive circuit unit configured to receive a gate signal from an optical transmission unit (91, 92) and to control the semiconductor switching device; characterized in further comprising a relay unit provided between the control circuit unit and the gate drive circuit unit, the relay unit having an optical transmission unit (41,44); a signal processing unit; and a power generating unit (43, 45), wherein the optical transmission unit (41, 44) is configured to receive the first signal, to input a corresponding signal into the signal processing unit, to receive a signal from the signal processing unit and to send out the gate signal, and the power generating unit (43, 45) is configured to receive power from a control device AC power source through a first transformer, to supply AC power to the gate drive circuit unit through a second transformer (71, 46, 55), and to supply power to the signal processing unit, and an optical transmission unit (41,44); a signal processing unit; and a power generating unit (43, 45), wherein the optical transmission unit (41, 44) is configured to receive the first signal, to input a corresponding signal into the signal processing unit, to receive a signal from the signal processing unit and to send out the gate signal, and the power generating unit (43, 45) is configured to receive power from a control device AC power source through a first transformer, to supply AC power to the gate drive circuit unit through a second transformer (71, 46, 55), and to supply power to the signal processing unit, and the relay unit, the gate drive circuit unit and the semiconductor switching device are accommodated in an accommodating case that is made of conductive material, and the accommodating case is connected to a middle electric potential terminal having a middle electric potential of said inverter main circuit voltage between said terminals (P, N).