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 to 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.