Abstract: A method for testing leakage of a pipe passage employs a flow rate of gas used in testing supplied to the inside of a pipe passage undergoing testing hermetically sealed on one side while detecting flow rate with a flow measuring device and pressure with a pressure detector, and detecting temperature of the gas used, and inputting the detected values of pressure, flow rate and temperature into a computation treatment apparatus, and internal capacity VL of the pipe passage is computed as VL=(supplied flow rate Q×pressure applied time ?t)/(pressure rise value ?P2), and next, the volume QL leaked from the pipe passage is computed as QL=(pressure drop value ?P2?×internal capacity VL)/(pressure drop time ?t?).