A switch failure diagnosis device for using in a current path between an electric device (6,7) and an electric storage device, the switch failure diagnosis device comprising: a plurality of switches (31A, 31B) connected parallel to each other in the current path between the electric device and the electric storage device; and a switch terminal voltage detector (35, 36) configured to output a switch terminal voltage detection signal corresponding to a voltage across the switches; characterised in that the switch failure diagnosis device further comprises a controller configured to: select (S21, S41) the switches at different time in sequence and input (S22, S45) an open instruction signal to each switch at the time when the switch is selected; and determine in an close failure diagnosis process, based on the switch terminal voltage detection signal output (V3B) while the open instruction signal is given to the selected switch, that at least one of the switches has a failure if the detected voltage across the switches is in a failure determination range; determine (S11, S13) whether the electric device is in an energized state or a non-energized state; perform an open failure diagnosis process for determining whether at least one of the switches has a failure if the electric device is determined to be in the non-energized state; and perform the inputting of the open instruction signal to the selected switch and the close failure diagnosis process for determining whether at least one of the switches has a failure if the electric device is determined (S13:YES) to be in the energized state and no failure of switches is determined (S13:NO) while the electric device is in the non-energized state. a plurality of switches (31A, 31B) connected parallel to each other in the current path between the electric device and the electric storage device; and a switch terminal voltage detector (35, 36) configured to output a switch terminal voltage detection signal corresponding to a voltage across the switches; characterised in that the switch failure diagnosis device further comprises a controller configured to: select (S21, S41) the switches at different time in sequence and input (S22, S45) an open instruction signal to each switch at the time when the switch is selected; and determine in an close failure diagnosis process, based on the switch terminal voltage detection signal output (V3B) while the open instruction signal is given to the selected switch, that at least one of the switches has a failure if the detected voltage across the switches is in a failure determination range; determine (S11, S13) whether the electric device is in an energized state or a non-energized state; perform an open failure diagnosis process for determining whether at least one of the switches has a failure if the electric device is determined to be in the non-energized state; and perform the inputting of the open instruction signal to the selected switch and the close failure diagnosis process for determining whether at least one of the switches has a failure if the electric device is determined (S13:YES) to be in the energized state and no failure of switches is determined (S13:NO) while the electric device is in the non-energized state.