If input signal is transmitted to a main body unit (10), a sensor amplifier (30) stores the input signal in itself as measurement data (S01), and transmits the input signal with the added transfer order information to the main body unit (10) (S02). By checking the transfer order information added to the input signal, the main body unit (10) can confirm if the input signal is deficient or not. Upon the end of the measurement of the physical quantity (S03), the main body unit (10) transmits the retransmission request that requests the retransmission of the deficient portion of the data to the sensor amplifier (30) (S04). According to the retransmission request from the main body unit (10), the sensor amplifier (30) extracts the deficient portion of the input signal data stored in itself (S05) and retransmits the extracted input signal data to the main body unit (10) (S06).