An electrophoresis apparatus having a plate on which channels are formed, a tray on which the plate is to be set, a tray driving unit for rotary driving the tray on which the plate is set, a voltage application unit for applying voltage to a buffer agent in the channels on the plate, an optical detection part for irradiating the channels on the plate with light, and detecting fluorescence that is generated from the sample due to the light irradiation. A plate clamp pressing the plate at only the channel formation areas of the plate against the plate setting surface of the tray, thereby fixing and holding the plate on the plate setting surface of the tray.