To restrict contamination in a clean space by a method wherein, in a charged particle beams apparatus in which a main body is arranged at an external space side, an opening for an entrance and exit of a sample is provided in a partition wall separating the clean space from the external space.

A clean room 50 is defined from the external space by a partition wall 51, and the interior of the clean room 50 is cleaned to a high degrees by an air cleaner 57, or a clean air in which temperatures and humidities are regulated fixedly is circulated. Furthermore, there are provided gateways 52, 53 for carrying in and out an apparatus, etc., and for being used for an entrance and exit of operators. These gateways are provided with double doors 52a, 53a, respectively, and a slightly high pressure is maintained for the external space in the clean room 50 so that an air including dusts does not flow in the clean room from the outside via clearances of the entrance and exit or the partition wall 51. In charged particle beams apparatus 55a, 55b, 55c installed in the clean room, a main body is arranged outside the clean room 50, and only a sample acceptance part is connected to a clean space inside the clean room via an opening part provided in the partition wall 51.