To enable measurement of insulation resistance using a common power supply for load signal measurement and insulation resistance measurement in the middle of operation time even when the operation time is long.

Power is supplied by a power supply 40, via two power supply excitation terminals 32, 34, to two power supply excitation ends 10a, 10b of a bridge circuit 10 in which strain gauges 6 attached to a metallic strain element 2 are connected to each other, and measurement terminals 36, 38 are connected to two output ends 10c, 10d of the bridge circuit 10. One terminal of a reference resistor 50 is connected to the metallic strain element 2 or a conductor 2a in contact with the strain element 2, and a current flow path is formed so that a current from the power supply 40 is made to flow through the reference resistor 50 via an insulation resistance components present between the strain gauges 10, the power supply excitation ends 10a, 10b and the output ends 10c, 10d and the metallic strain element 2 or the conductor 2a and return to the power supply 40, and voltage produced between both ends of the reference resistor 50 is measured by an operational amplifier 64.