PROBLEM TO BE SOLVED: To provide a failure diagnosis device capable of diagnosing at which load the failures occur by a simple circuit configuration in the case that two loads are connected in series.SOLUTION: A failure diagnosis device comprises: a power supply circuit 13 supplying a current to two loads 11 and 12 connected in series; an output voltage detector 14 having two resistors R1 and R2 connected in series, detecting an output voltage of the power supply circuit 13, and dividing the detected voltage by the resistors R1 and R2 and outputting it; an intermediate voltage detector 15 having two resistors R3 and R4 connected in series, detecting an intermediate voltage of the loads 11 and 12, and dividing the detected voltage by the resistors R3 and R4 and outputting it; and a determination part storing a reference output voltage at the time when the voltage division is performed at the output voltage detector 14 and a reference intermediate voltage at the time when the voltage division is performed at th
