A signal processing apparatus includes an interface module and a signal processing module. The interface module includes an identification resistance for identifying the interface module, the interface module being connectable to a device configured to perform at least one of measuring of a measuring target and operating of an operation target. The signal processing module includes a first connection terminal connected to one end of the identification resistance, a first power source connected to the identification resistance via the first connection terminal, a detector configured to detect any one of voltage and electrical current at the first connection terminal, and a signal processor configured to process signals received from and transmitted to the device.