To provide an insulation state determination device capable of reducing cost.

First wiring 6 and 7 connecting an inverter 5 with a fuel battery 3 are interposed by a first relay 10 and a second relay 11 respectively. A second wiring 8 is connected with the first wiring 6 at a point closer to the inverter 5 than to the first relay 10. A second wiring 9 is connected with the first wiring 7 at a point closer to the inverter 5 than to the second relay 11. The second wiring 8 and 9 are interposed by a third relay 12 and a fourth relay 13 respectively. An insulation resistance detection circuit 14 is connected with a secondary battery 4 and an earth. The first relay 10, the second relay 11, the third relay 12 and the fourth relay 13 are turned on/off and, on the basis of an insulation resistance value which is outputted from the insulation resistance detection circuit 14, insulation states of the fuel battery 3 and the secondary battery 4 are determined.