To enable a semiconductor pressure sensor to be manufactured at a low cost while abolishing a glass seating in the constitution of forming a reference pressure chamber in a single-crystal silicon substrate in a sealed state.

After a small opening part 19 is formed at a predetermined location in an insulating film 18 formed on the back surface of an Si substrate 12, an alkali anisotropic etching is performed. As etching speed on a (111) surface is extremely slow in the alkali anisotropic etching, small cavities in a shape surrounding the small opening part 19 along the (111) surface are formed in a connected state, and an edge part appears, and etching the edge part on a (311) surface, small cavities with the (111) surface as a wall surface are formed again. Finally, a cavity 17 corresponding to an envelope surrounding the overall small opening part 19 with the (111) surface is formed, and an Si diaphragm 13 for detecting pressure is formed on the opposite side. Then by sealing the small opening part 19 with a thin film 20, a reference pressure chamber 21 is formed.