A switch failure diagnosis device for using in a current path between an electric device and an electric storage device includes plural switches, a switch terminal voltage detector, and a controller. The switches are connected parallel to each other in the current path. The switch terminal voltage detector outputs a switch terminal voltage detection signal. The controller is configured to: select the switches at different time in sequence and input an open instruction signal to each switch at the time when the switch is selected; and determine, based on the switch terminal voltage detection signal output while the open instruction signal is given, that at least one of the switches has a failure if the detected voltage is in a failure determination range.