A quadrupole accelerator includes a center member, a first side member, and a second side member. The center member includes a center outer frame part, a first electrode and a second electrode. The first side member includes a first side outer frame part, a first wall part and a third electrode. The second side member includes a second side outer frame part which extends from the second side outer frame part toward an outside, a second wall part and a fourth electrode. The center member is formed seamlessly. The first side member is formed seamlessly. The second side member is formed seamlessly. The first side outer frame is fixed to a first side of the center outer frame part by a first fixing member. The second side outer frame is fixed to a second side of the center outer frame part by a second fixing member.