In the SPring-8 storage ring, we could not make an accurate measurement of pressure using a Bayard-Alpert vacuum gauge near the photon-absorber due to the irradiation of the scattered X-ray to a gauge head. Therefore the effect of X-ray irradiation to a gauge head has been investigated experimentally. As the results, the ion current of the collector electrode was detected without emission current from the filament. It was confirmed that the causes of the ion current were photoelectric effect and Compton scattering to the collector electrode and the ionization of gases. Especially, the ion current caused by ionization of air between the gauge head and the connector was extremely large. With these experiments, we identified main cause and its mechanism.