PURPOSE: To prevent the freezing of moisture so as to operate a head valve always satisfactorily.

CONSTITUTION: An annular protruding edge part 18 is circumferentially provided on the upper part outside of a cylindrical impact cylinder 3a. A head valve 6 of short cylinder shape is disposed at the upper part of the protruding edge part 18, coaxially with the impact cylinder 3a in such a way as to be vertically slidable, and an air chamber 2 stored with compressed air is formed on the outside of the head valve 6. When the lower end of the head valve 6 is separated from the protruding edge part 18, the air chamber 2 is opened to the impact cylinder 3a, and when the lower end of the head valve 6 comes in contact with the protruding edge part 18, the air chamber 2 is closed to the impact cylinder 3a to seal. In a nailing machine of such constitution, the protruding edge part 18 of the impact cylinder 3a and the external wall surface of the upper part are covered with an integral rubber cover 19.