A film forming apparatus for forming a film on a substrate by using a magnetron sputtering method. The film forming apparatus includes: a substrate holder configured to hold a substrate; a target holder configured to hold a target made of a ferromagnetic material to face the substrate holder; a magnet provided on a surface of the target holder opposite to the substrate holder, and configured to leak a magnetic field to a front surface of the target held by the target holder that is a surface close to the substrate holder; and a magnetic field strength measurement device configured to measure a strength of the magnetic field.