To provide a flow meter capable of preventing failure caused by cloud-to-ground discharge and normally and continuously operating.

A ground potential E is measured. The presence or absence of the approach of thunder is determined on the basis of the ground potential E, and the grounding state of a communication line 30 is changed over. The approach of the thunder is recognized on the basis of a reduction phenomenon in the ground potential E to automatically ground the line 30. In this case, even when an excess current passes the line 30 at the occurrence of cloud-to-ground discharge, the excess current is induced to the ground via a wire W3 without reaching a microcomputer 14. Therefore, the microcomputer 14 doe not fail regardless of the size of the cloud-to-ground discharge, contrary to a conventional gas meter provided with a varistor. Neither the microcomputer 14 nor a casing 11 and the other constituent elements housed inside the casing 11 do not fail.