To provide a photo cathode device with high quantum efficiency, a long service life, and a low manufacturing cost.

An ultraviolet laser beam UV into a quartz prism 1 enters an aluminum layer 2 with an incident angle in the photo cathode device. A surface-plasmon resonant light is excited by the ultraviolet laser beam UV at the aluminum layer 2; and a photo electron is emitted from a photoelectric surface opposite to the surface of the quartz prism 1 at the aluminum layer 2, by means of the photoelectric effect of the surface-plasmon resonant light. The incident-light angle amounts to a plasmon-dip angle with the minimal reflectance R of the ultraviolet laser beam UV at the light-incident surface of the aluminum layer 2 in a total-reflection region.