The object is to provide an occupant detection device that can accurately detect whether there is a passenger, even when a personal computer, a tablet terminal, a smartphone or the like is placed on a vehicle seat. The occupant detection device is provided for a vehicle including a vehicle body, a vehicle seat and an in-vehicle battery. The occupant detection device includes: an electrostatic sensor provided in the seat and configured to measure capacitance between the sensor and the vehicle body; an interrupting unit configured to interrupt a function of the in-vehicle battery; a controller configured to temporarily activate the interrupting unit when a measured value of the electrostatic sensor is higher than a predetermined threshold; and a judgement unit configured to judge that there is a passenger when the measured value of the electrostatic sensor is higher than the predetermined threshold even while the interrupting unit is activated.