Provided is a gas circuit breaker wherein an abnormality is detected when water enters the gas circuit breaker. The gas circuit breaker is provided with a flow quantity detecting means (8) which measures a flow quantity, and based on a detected value, an instantaneous flow quantity is calculated by means of a flow quantity calculating means (15). Signal amplification adjusted by the flow quantity detecting means (8) is judged by an amplification judging means (18). When amplification is a predetermined value or more, timing is started, and measurement conditions of the flow quantity detecting means (8) are set by means of a measurement condition setting means (16) based on the flow quantity obtained by the flow quantity calculating means (15). A measurement ratio calculating means (20) obtains a measurement condition ratio while measurement is performed, based on values obtained from a time measuring means (19) and the measurement condition setting means (16), and when a measurement ratio is a predetermined ratio or more, it is judged that the flow quantity detecting means (8) is abnormal, and gas supply is shut off by a shutoff means (22).