To provide a volume calculation method or the like which allows the volume calculation of target space without relying on the volume value of a tank calculated by measurement.

A gas concentration (a first gas concentration (C1)) of a concentration measuring gas when a predetermined amount of the concentration measuring gas is injected into the target space At is measured. A known volume body, of which the volume (Vb) is determined in advance, is input into the target space At. A gas concentration (a second gas concentration (C2)) of the concentration measuring gas when a predetermined amount of the concentration measuring gas is injected into the target space At is measured. Using the formula, [VxC1=(Vx-Vb)C2], the spatial volume (Vx) of the target space At is calculated.