According to the present invention, mistaken detection of a gas leak can be prevented even when using an appliance which has been newly installed in a dwelling receiving a gas supply, and whereby a gas leak can be detected rapidly, efficiently and accurately. The characteristics extraction means 5 extracts characteristics of a gas flow including a combination of the instantaneous flow volume data and the instantaneous flow volume time differential value, on the basis of the data obtained by the flow volume measurement means 1, pressure measurement means 2, instantaneous flow volume time differential operation means 3 and pressure time differential operation means 4. The leak detection means 7 compares the characteristics data for respective types of gas appliance or for a gas leak registered in the storage means 6 with the characteristics extracted by the characteristics extraction means 5, and if a matching is not achieved, further judges whether there is a flow volume variation or nozzle variation, and whereby it judges whether there is a gas leak or whether an unregistered appliance is in use. If occurrence of a gas leak, the warning means 9 issues a warning. If an unregistered appliance is in use, the characteristics data forming the basis of the judgment is registered as a new characteristics data in the storage means 6 by the characteristics data registration means 8.