An object is to provide a cooling unit which can reduce influence of radiation heat from a work piece having high temperature on members surrounding the work piece, prevent leak of coolant and vacuum leak, reduce cost, and prevent turning angle of a work piece conveying mechanism from being limited. The cooling unit is attached to the work piece conveying mechanism in the state that the outer wall part is in close contact with the to-be-cooled surface. The coolant stored in the lower space is evaporated by the heat transmitted from the to-be-cooled surface P via the outer wall part, and the to-be-cooled surface is cooled via the outer wall part by the heat of evaporation lost at the time of the evaporation of the coolant. The vapor in the lower space (coolant container) is discharged to the vacuum chamber by the vapor exhaust unit when the pressure of the vapor in the lower space reaches the fixed value or higher.