A photocathode high-frequency electron-gun cavity apparatus structured with a BNL type high-frequency acceleration cavity, comprising: a high-frequency acceleration cavity formed by connecting a plurality of cavity cells (5, 6), each having a curved surface shape formed with any of a trigonometric-function curved line, an arc, an oval, and a parabola so that the cavity cells (5, 6) are formed to have a smooth and curved surface shape approaching to a standing wave waveform suitable for an acceleration high-frequency high-electric field; a photocathode which is arranged at an end part of the cavity cells (5,6) of the high-frequency acceleration cavity; an electron beam extraction port arranged at the other end part of the cavity cells (5, 6) for extracting an electron beam generated and accelerated in the high-frequency accelerated cavity, a laser entering port which is arranged at a position facing the photocathode at the same end part of the cavity cells (5, 6) as the electron beam extraction port and behind the electron beam extraction port of the high-frequency acceleration cavity and which is used for entering of laser light suppliable to the photocathode into the high-frequency acceleration cavity; a high-frequency power input coupler port which is arranged at a side part of the high-frequency acceleration cavity and which is used for inputting high-frequency power into the high-frequency acceleration cavity; a vacuum exhaust port which is arranged at a side part of the high-frequency acceleration cavity and which is used for vacuum exhausting of the high-frequency acceleration cavity; and a high-frequency resonant tuner which is arranged at a side part of the high-frequency acceleration cavity and which is used for adjusting a high-frequency resonance frequency of the cavity cell, wherein the cavity cells (5, 6) are formed only with a curved surface shape without having a sharp angle part, each curved surface shape at the vicinity of a top section of each cavity cell (5, 6) and an iris is formed with an arc and a curvature radius R in a range between 5 and 20 mm inclusive, and a surface roughness of the inner face of the cavity cells (5, 6) is 0.05 μm or less. a high-frequency acceleration cavity formed by connecting a plurality of cavity cells (5, 6), each having a curved surface shape formed with any of a trigonometric-function curved line, an arc, an oval, and a parabola so that the cavity cells (5, 6) are formed to have a smooth and curved surface shape approaching to a standing wave waveform suitable for an acceleration high-frequency high-electric field; a photocathode which is arranged at an end part of the cavity cells (5,6) of the high-frequency acceleration cavity; an electron beam extraction port arranged at the other end part of the cavity cells (5, 6) for extracting an electron beam generated and accelerated in the high-frequency accelerated cavity, a laser entering port which is arranged at a position facing the photocathode at the same end part of the cavity cells (5, 6) as the electron beam extraction port and behind the electron beam extraction port of the high-frequency acceleration cavity and which is used for entering of laser light suppliable to the photocathode into the high-frequency acceleration cavity; a high-frequency power input coupler port which is arranged at a side part of the high-frequency acceleration cavity and which is used for inputting high-frequency power into the high-frequency acceleration cavity; a vacuum exhaust port which is arranged at a side part of the high-frequency acceleration cavity and which is used for vacuum exhausting of the high-frequency acceleration cavity; and a high-frequency resonant tuner which is arranged at a side part of the high-frequency acceleration cavity and which is used for adjusting a high-frequency resonance frequency of the cavity cell, wherein the cavity cells (5, 6) are formed only with a curved surface shape without having a sharp angle part, each curved surface shape at the vicinity of a top section of each cavity cell (5, 6) and an iris is formed with an arc and a curvature radius R in a range between 5 and 20 mm inclusive, and a surface roughness of the inner face of the cavity cells (5, 6) is 0.05 μm or less.