A leak detecting method for vessels in which a gas of a given pressure is mixed with sulfur hexafluoride of a given partial pressure and the mixture is introduced into a vessel, and a gas in the vicinity of an outer peripheral surface of the vessel is sucked by a gas sucking mechanism.According to one form of the invention, the sucked air is introduced into an absorption cell and a P(16)-line laser light of a carbon dioxide gas laser is introduced into the absorption cell. Then, the presence or absence of absorption of the P(16)-line laser light within the absorption cell is detected in accordance with a change in the transmitted light intensity of the P(16)-line laser light and the presence or absence of a leak in the pressure vessel is discriminated in accordance with a result of the detection. In accordance with another form of the invention the sucked air is introduced into a multiple reflection absorption cell and a P(16)-line laser light of a carbon dioxide gas laser and a reference light slightly different in wavelength from the P(16)-line laser light are introduced into the multiple reflection absorption cell. Then, the presence or absence of absorption of the P(16)-line laser light within the multiple reflection absorption cell is detected in accordance with a change in a light intensity ratio between the reflected lights of the P(16)-line laser light and the reference light, and the presence or absence of a leak in the pressure vessel is discriminated in accordance with a result of the detection.