One embodiment of a particle accelerator includes: a vacuum container with its inside evacuated to produce vacuum, the vacuum container being formed with a laser beam entrance window for allowing a laser beam to enter; a target arranged in the vacuum container so as to be irradiated with a laser beam to generate ions; and a condenser lens for focusing the laser beam onto the target. The condenser lens is arranged at the laser beam entrance window of the vacuum container, and takes a role of a vacuum bulkhead.