An occupant weight detecting device includes plural load detecting device and a signal processing device. The plural load detecting device outputs a load signal in response to largeness of load applied to the seat, and the signal processing device sets an occupant load detection range. The signal processing device outputs a variable primary load detection signal based on the load signal in case of the load signal being within the occupant load detection range and a constant secondary load detection signal in case of the load signal being out of the occupant load detection range. The occupant weight detecting device further includes a control device to which the primary load detection signal or the secondary load detection signal is inputted. The control device determines that error is generated on the load detecting device when the signal processing device continues to output the secondary load detection signal during predetermined time.