To provide a flow-rate measuring apparatus and a leakage detector, improving leakage detection accuracy, by accurately detecting gas leakages, without begin influence by the temperature of a gas.

A first counting means 5a-3 starts counting of a first prescribed time, while a usage judging means 5a-2 is being judged 'no'. A leakage detecting means 5a-4 detects the gas leakage, when {gas pressure (hereinafter to be referred to as Pa) detected by the first counting means/gas temperature (hereinafter to be referred to as T1), detected at the counting starting time}≠{gas pressure (hereinafter to be referred to as P2) detected by the first counting means at count ending time/gas temperature (hereinafter to be referred to as T2) detected at counting ending time} is satisfied.