To provide a seal structure of a valve and a valve mounting part stabilizing fixing of the valve and a planar member and manufacturable at a low cost.

This valve 10 has a double seal structure by a metal seal and an O-ring 35 to improve sealing property as compared with conventional sealing property. The fixing of the valve 10 is stabilized since the planar member 33 is directly held by an outside projecting wall 26 formed on the valve 10 and an annular fitting 30. The O-ring 35 is pressed by a tapered part 27 to be enlarged in diameter and deformed and is closely adhered to the tapered part 27 and the planar member 33, and thereby a degree of freedom of the size of the O-ring 35 is increased and working accuracy is not required like an O-ring groove to suppress working cost.