A leak detecting method and apparatus for a gas pipe line in which the inflow rate Qin or the outflow rate Qout is measured from instant to instant at each of all the inlet and outlet ports of the gas pipe line and the presence or absence of a gas leak is determined in accordance with the measured values. The measured inflow rates Qin and outflow rates Qout are respectively integrated over a given period of time and the occurrence of a gas leak is determined when the sum .SIGMA..intg.Qdt of the integrated values deviates from a predetermined value. On that occasion, in accordance with the data obtained by these calculations the relation between the integration time and the standard deviation of .intg.Qindt-.intg.Qoutdt/.intg.Qindt is calculated so that in accordance with the relation an integration time T corresponding to a rate of leakage to be detected is determined and the integration over time is performed from each of a plurality of time instants shifted from each other by an integral fraction of the integration time T whereby the above-mentioned determination is made each time the integration time T is reached.