PURPOSE: To uniformly cool a spectroscopic element body by providing inside a supplementary coolant medium flow path of tower shape cross section extending from one end to the other end so that the tower part comes in between each coolant medium flow path.

CONSTITUTION: In a spectroscopic element body 19, a plurality of coolant medium flow paths 20 extending in parallel from the one end to the other end and a supplementary coolant medium flow path 21 are provided. The flow path 21 having tower shape (tear drop shape) cross section is provided so that the tower parts 21s come in between the flow paths 20 when viewed from one surface side of the body 19. When radiation light beam is introduced in the reflection surface 19r of the body 19 in this state, the coolant media continuously flowing in the flow paths 20 and 21 absorb the heat given to the body 19 and uniformly cool the reflection surface 19r of the body 19.