PURPOSE: To eliminate the consumption of a power supply even in case a gas flow rate signal keeps a fixed positive or negative level and to realize the long-period use of a power supply battery by using a gas flow rate detecting means which conducts only at the change of the gas flow rate signal.

CONSTITUTION: A reed switch 12 is opened and closed in accordance with the rotation of a rotary plate 11 of a gas meter device 1. Thus the voltage having its ON and OFF periods equal to each other is applied to a gas flow rate detecting circuit 8. A base current flows instantaneously to resistances 83 and 84 at an instance when the switch 12 is turned on. Then the both-end voltage of a resistance 13 is inputted to a logic circuit 4 via an inverter 9 having the threshold value and informed. The gas flow rate is equal to zero when the switch 12 is turned on. If this state lasts, a capacitor 81 is kept at the level of the battery voltage and no base bias is applied to a transistor 85. Thus the transistor 85 never conducts. Then the transistor 85 does not conduct neither in case the OFF state of the switch 12 is kept.