Abstract: An apparatus detects a leakage position between the upstream and downstream points of a fluid passage, where the fluid leaks. The flow rate of the fluid is detected both at the upstream and downstream points repeatedly at intervals much shorter than a difference .DELTA..tau. between time .tau..sub.1 and time .tau..sub.2 which elapse until a flow rate change appears respectively at the upstream and downstream points. Thus the pattern of the flow rate variation is detected with respect to the time .tau..sub.1 and the time .tau..sub.2, thereby obtaining the time difference .DELTA..tau.. Based on the time difference .DELTA..tau., the distance x between the upstream and downstream points is obtained, thus locating the fluid leakage position, by the following formula: ##EQU1## where l denotes the distance between the upstream and downstream points, and a the speed of sound through the fluid.