A gas amount measuring device includes a vacuum container (1) filled with a molten metal, a first pipe (5a) being attached to the vacuum container (1), an air release valve (4) being attached to the first pipe (5a), a second pipe (5b) being attached to the vacuum container (1), a calibration gas generator (6) being attached to the second pipe (5b), a third pipe (5c) being attached to the vacuum container (1), a vacuum gauge (13), a first valve (11a), a leak valve (12), a second valve (11b), a turbomolecular pump (14), a third valve (11c), and a roughing pump (15) being attached to the third pipe (5c) in order from a vacuum container (1) side, a first bypass pipe (16) connecting the third pipe (5c) between the vacuum container (1) and the first valve (11a) to the third pipe (5c) between the leak valve (12) and the second valve (11b), a fourth valve (11d) being attached to the first bypass pipe (16), and a second bypass pipe (17) connecting the third pipe (5c) between the leak valve (12) and the second valve (11b) to the third pipe (5c) between the third valve (11c) and the roughing pump (15), a fifth valve (11e) being attached to the second bypass pipe (17).