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 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 ; a signal processing unit ; and a power generating unit , wherein the optical transmission unit 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 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 , 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 (M) 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 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 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 ; a signal processing unit ; and a power generating unit , wherein the optical transmission unit 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 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 , and to supply power to the signal processing unit , and an optical transmission unit ; a signal processing unit ; and a power generating unit , wherein the optical transmission unit 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 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 , 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 (M) having a middle electric potential of said inverter main circuit voltage between said terminals (P, N). <EOS>