To provide an air injection method and a valve for air injection by which air can be surely injected to the inside of a pipe line when a valve is closed.

In a spring type air valve 4 usually arranged so as to communicate to a pipe line 2, even if a valve element is operated to be opened and closed with a float which moves up and down according to a water level in a valve casing, air is exhausted or sucked. When inside pressure of the pipe line detected by a sensor means turns negative, air is injected in the pipe line by forcefully operating the valve element to be opened by oil pressure or air pressure supplied from the driving source in an oil type air valve 40 usually arranged so as to communicate with the pipe line 2.