A connection diagnostic apparatus including an electric circuit and a ground fault detector, wherein said ground fault detector includes an oscillator for generating a pulse signal, a coupling capacitor connected to said electric circuit, and a voltage detector for detecting a voltage value of the pulse signal between the oscillator and the coupling capacitor, and wherein said electric circuit includes a first connection line for connecting a positive electrode of a power supply and a load and a second connection line for connecting a negative electrode of the power supply and the load with: a first relay provided in the first connection line; and a second relay provided in the second connection line; characterized in that the connection diagnostic apparatus further includes a programmable controller programmed to: turn on and off the first relay, determine that the ground fault detector is not normally connected to the electric circuit via the first relay if either a temporary voltage decrease amount (ΔV R+ON) from a reference voltage value (V1 DEF) detected by the voltage detector when the first relay is turned on or a temporary voltage increase amount (ΔV R+OFF) from the reference voltage value (V1 DEF) detected by the voltage detector when the first relay is turned off is smaller than a predetermined determination value (ΔV), turn on and off the second relay, and determine that the ground fault detector is not normally connected to the electric circuit via the second relay if either a temporary voltage decrease amount (ΔV R-ON) from the reference voltage value (V1 DEF) detected by the voltage detector when the second relay is turned on or a temporary voltage increase amount (ΔV R-OFF) from the reference voltage value (V1 DEF) detected by the voltage detector when the second relay is turned off is smaller than the predetermined determination value (ΔV). a first relay provided in the first connection line; and a second relay provided in the second connection line; characterized in that the connection diagnostic apparatus further includes a programmable controller programmed to: turn on and off the first relay, determine that the ground fault detector is not normally connected to the electric circuit via the first relay if either a temporary voltage decrease amount (ΔV R+ON) from a reference voltage value (V1 DEF) detected by the voltage detector when the first relay is turned on or a temporary voltage increase amount (ΔV R+OFF) from the reference voltage value (V1 DEF) detected by the voltage detector when the first relay is turned off is smaller than a predetermined determination value (ΔV), turn on and off the second relay, and determine that the ground fault detector is not normally connected to the electric circuit via the second relay if either a temporary voltage decrease amount (ΔV R-ON) from the reference voltage value (V1 DEF) detected by the voltage detector when the second relay is turned on or a temporary voltage increase amount (ΔV R-OFF) from the reference voltage value (V1 DEF) detected by the voltage detector when the second relay is turned off is smaller than the predetermined determination value (ΔV). turn on and off the first relay, determine that the ground fault detector is not normally connected to the electric circuit via the first relay if either a temporary voltage decrease amount (ΔV R+ON) from a reference voltage value (V1 DEF) detected by the voltage detector when the first relay is turned on or a temporary voltage increase amount (ΔV R+OFF) from the reference voltage value (V1 DEF) detected by the voltage detector when the first relay is turned off is smaller than a predetermined determination value (ΔV), turn on and off the second relay, and determine that the ground fault detector is not normally connected to the electric circuit via the second relay if either a temporary voltage decrease amount (ΔV R-ON) from the reference voltage value (V1 DEF) detected by the voltage detector when the second relay is turned on or a temporary voltage increase amount (ΔV R-OFF) from the reference voltage value (V1 DEF) detected by the voltage detector when the second relay is turned off is smaller than the predetermined determination value (ΔV).