To use one common power source for both measuring a load signal and measuring an insulation resistance value.

Two opposing excitation terminals 20a, 20b of a bridge circuit 18 which is constituted by a plurality of strain gauge 8 for measuring the load provided in a strain inducing portion 6 of a metallic strain inducing body 2 are connected to a common line C and a normal line N of a power source 38. The output voltage between output terminals 20c, 20d of the bridge circuit 18 is changed by the application of the load to the metallic strain inducing body 2. A strain gage 24 for measuring the insulation resistance value is provided so as to apply the same environment load as that of each strain gage 8 for measuring the load to the strain inducing body 2, and a bridge circuit 49 including the strain gage 24 for measuring the insulation resistance value is connected to the common line C and the normal line N of the power source 38. The output voltage of the bridge circuit 49 is changed according to the resistance change of the strain gage 24 for the insulation resistance value.