PURPOSE: To prevent a back fire generated upon welding by providing a back fire preventing mechanism between an acetylene bomb and a welding tool or the like.

CONSTITUTION: Below a thumb screw 2 slidingly attached and constantly pressed upward by a support spring 7 between a main body 1 and the screw 2, the upper end part of an inner cylinder 9 slidingly attached in the main body 1 is disposed. A check valve 10 operating to open and close a feed passage 17 formed in the inner cylinder 9 is provided in the lower part of the inner cylinder 9. Then, a back fire preventing mechanism 8 actuates the check valve 10 to close the feed passage when a strong pressure due to a back fire is applied thereto, and the inner cylinder 9 slides upward to close a gas guide passage 14.