It is an object of the present invention to provide a metal diaphragm capable of achieving a higher strength, excellent corrosion resistance, and a smooth surface condition, and a pressure sensor including the diaphragm. The diaphragm according to the present invention includes a two-phase stainless steel having a composition of 24 to 26 mass% Cr, 2.5 to 3.5 mass% Mo, 5.5 to 7.5 mass% Ni, 0.03 mass% or less C, 0.08 to 0.3 mass% N, and the balance Fe and inevitable impurities, and having a 0.2% proof stress of 1300 MPa or higher.