FLOWMETER

PROBLEM TO BE SOLVED: To solve the problem that stop of flow rate measurement with deterioration of a remaining battery level (voltage) of a flowmeter occurs.SOLUTION: A flowmeter 10 includes a flow rate arithmetic unit 60 and an external communication unit 70. The flow rate arithmetic unit 60 drives a transmission side ultrasonic wave sensor 40, and includes a transmitter-receiver circuit 62 for receiving a detection signal from a reception side ultrasonic wave sensor 50, a flow rate arithmetic circuit 64, a first battery 66, a first voltage monitoring circuit 68, and a power source changeover switch 80. The external communication unit 70 includes a second battery 72, a communication circuit 74, a second voltage monitoring circuit 76, and a radio antenna 78. The first and second voltage monitoring circuits 68, 76 monitor the residual amounts (voltage) of the batteries 66, 72, and when any one of the residual amounts of the batteries 66, 72 becomes a prescribed value or less, supply the power of the other battery into the flow rate arithmetic unit 60 or the external communication unit 70.