To enable machining for a long time with a large output power without being affected by thermal distortion and lowering the laser machining accuracy and to provide an inexpensive lens hardly causing damage and having excellent durability by circulating lens fluid in a lens container.

A hollow where lens liquid 10 is formed is formed inside a lens container 1. The lens container 1 has an upper lens surface part 2, a lower lens surface part 3 and a cylindrical metallic frame 4. The lower lens surface part 3 and the upper lens surface part 2 are mounted on the metallic frame 4. An inlet port 5 and an outlet port 6 are formed on opposed positions between the upper lens surface part 2 and the lower lens surface part 3 on the side of the metallic frame 4. An inlet tube 7 and an outlet tube 8 are connected to the inlet in port 5 and the outlet port 6, respectively. The lens liquid 10 is allowed to flow into the lens container 1 from the inlet tube 7 through the inlet in port 5, and allowed to flow out from the outlet port 6 through the outlet port 6 to fill the laser container 1 with the lens liquid 10.