An earth fault detection circuit for detecting an earth fault between a high-voltage battery and a motor includes: an A.C. signal generation portion generating an A.C. signal; a first capacitive element between the A.C. signal generation portion and one end side of the high-voltage battery; a voltage dividing circuit dividing a voltage on the one end side of the high-voltage battery; an earth fault detection portion detecting the earth fault between the high-voltage battery and the motor based on an earth fault detection signal inputted thereto; and a second capacitive element inputting an A.C. component of the voltage, on the one end side of the high-voltage battery, which is obtained by the voltage division in the voltage dividing circuit as the earth fault detection signal to the earth fault detection portion.