In leak testing by an accumulation method, it is possible to reliably detect accumulated inspection gas, and the reliability of leak testing is enhanced by reduction in background or impurity gas. At least an openable/closeable part 29 of a capsule 20 is housed inside a chamber 10 of a leak testing apparatus 1. A test workpiece 9 is housed in the capsule 20. Gas inside the chamber 10 is evacuated by a vacuum pump 31. A detector 2 detects inspection gas contained in this gas. The capsule 20 is continuously sealed for an accumulation time Ta, the capsule 20 is thereafter opened, and a leak is evaluated on the basis of detection information after the opening by the detector 2.