There is disclosed an improved leak checking method which can accurately test a sealing performance of a substrate holder more than conventional leak check techniques. The leak checking method includes: holding a substrate with a substrate holder, the substrate holder including a first holding member and a second holding member, the second holding member having an opening through which a surface of the substrate is exposed; pressing a sealing projection of the second holding member against the surface of the substrate when holding the substrate with the substrate holder; covering the surface of the substrate, exposed through the opening, and the sealing projection with a sealing cap; forming a hermetic space between the sealing cap and the substrate holder; introducing a pressurized gas into the hermetic space; and detecting a decrease in pressure of the pressurized gas in the hermetic space.