A gas internal combustion type nailing machine including a striking cylinder for slidably accommodating a striking piston therein, a combustion chamber formed in an upper side and capable of being opened and closed, a compressor for supercharging compressed air into the combustion chamber, a fuel supply device for supplying a fuel gas into the combustion chamber, and a safety device for preventing the nailing machine from being driven for ignition when the supercharge pressure of the compressed air exceeds a set pressure value.