A housing case for a crystal oscillator monitors a film thickness of a thin film to be formed on a surface of the crystal oscillator, the monitoring being performed by measuring a resonance frequency during film formation in a vacuum atmosphere. The housing case has: a case main body having disposed on an upper surface thereof a plurality of first recessed parts each being capable of housing therein a crystal oscillator in a horizontal posture with a main surface thereof facing in an up-and-down direction; a first cap body detachably mounted on the case main body from an upper side thereof; and an engaging means for engaging the first cap body relative to the case main body. The engaging means is so constructed and arranged that, in a state in which the first cap body is engaged with the case main body, the crystal oscillators housed in the first recessed parts are restrained from jumping out of position.