Abstract: An air relief device for use in a liquid flow pipe comprises an outer housing fixed to a large-diameter air outlet formed in a portion of the pipe arranged at an elevation, an inner housing rotatably fitted snugly in the outer housing, a float accommodated in the inner housing, and a lid member having a valve port and adapted to be put on the inner housing. The outer and inner housings have through holes bored therein respectively and allowed to coincide exactly with each other. Fluid communication or severance of the fluid communication between the pipe and the inner housing can be obtained by rotating the inner housing to vary the relation in position between the through holes in the outer and inner housings.