a chamber;

a wall member, partially placed in an internal space of the chamber, extending from the internal space toward an outside of the chamber to be exposed to a space at the outside of the chamber;

one or more insulating members provided on the wall member; and

a ground member which is made of silicon, provided in the internal space such that the ground member is not exposed to the space at the outside of the chamber and mounted on the one or more insulating members, the ground member as well as the wall member being set to have a ground potential,

wherein the wall member is configured to support the ground member in a non-contact state with the one or more insulating members therebetween,

each of the one or more insulating members has a spherical surface, and

the ground member is in contact with the spherical surface and mounted on the spherical surface,

wherein the one or more insulating members are multiple spherical bodies,

wherein the wall member is provided with multiple grooves opened upwards, and

each of the multiple spherical bodies is partially located in a corresponding groove among the multiple grooves,

wherein a respective bottom surface of each of the multiple grooves has a spherical concave surface and a flat surface surrounding the spherical concave surface, and

each of the multiple spherical bodies is partially inserted into the spherical concave surface in the corresponding groove.