To provide a power supply unit with a capacitor life diagnosis device which enables diagnosis in an ordinary operation state and to provide the capacitor life diagnosis device.

This power supply unit with a capacitor life diagnosis device comprises a capacitor built-in apparatus, which has a rectifier circuit 3 rectifying a supplied AC power, an input smoothing capacitor 4, and a switching circuit 7, and a capacitor life diagnosis device, which decides a missing capacity of the capacitor 4. The capacitor built-in apparatus has a switch 6 which cuts off the rectified output of the rectifier circuit 3. The capacitor life diagnosis device has a voltage detection unit 14, which detects that a voltage between both the ends of the capacitor 4 is lowered to a reference voltage, and a CPU 15 which decides whether a time from the cut-off of the switch 6 to a time when the voltage between both the ends of the capacitor 4 is not longer than a certain time or not.