To provide a laser beam machining apparatus with which a laser beam which permeates an object lens can be stably and easily focused on an object member of machining.

In the laser beam machining apparatus 1, the object lens 5 which is supported by a lens supporting part 4 is arranged on the optical path of a laser beam emitted from a light source 2. The lens supporting part 4 has a bowl-shaped supporting member 41 and a porous member 43 which is composed of a porous material fixed to its open end face and an air pass 4a by which the air is conducted from an air supply part to the porous member 43 is formed on the supporting member 41. When the air is supplied from an air hose 44 connected to the air pass 4a, the air is jetted downward from the porous member 43, the lens supporting part 4 is floated and the height of the lens supporting part 4 is held stably by discharging the air with an air discharging hose 45 connected to the inside space of the supporting member 41. In this way, the laser beam which permeates the object lens 5 is stably and easily focused on a glass 6.