The pneumatic tool has a check valve placed inside that includes an internal and external valve base and traveling valve. The internal valve base has an inlet hole, and an enlarged slot is formed on the external end of the inlet hole. An annular oblique edge is formed between the enlarged slot and inlet hole, and an air hole is placed on the enlarged slot. The external valve base has an outlet hole that is connected to the outside of the pneumatic tool. A protruding tube is formed inside the outlet hole. Interval space is formed between the inside of the protruding tube and the inlet hole for the internal valve base; and the traveling valve can be traveling inside the interval space. The built-in check valve can avoid external force, which extends the shelf life and makes appearance of the product look neater and practical.