A pressure gauge for measuring the pressure of a fluid flowing through a pipe, comprising a stainless steel pressure gauge body having a straight cylindrical internal space passing there through, the pressure gauge body comprising: a cut away part providing a flat surface sufficiently polished to serve as a strain-measurement strain gauge attaching surface, including: a strain-measurement strain gauge attaching surface on the outside surface of the pressure gauge on said flat surface, at a portion having a thickness where pressure produced in the internal space when a fluid is made to flow through the internal space can be substantially detected; and a portion other than the strain-measurement strain gauge attaching surface, the portion having a thickness where the pressure produced in the internal space is substantially impossible to detect; said pressure gauge further comprising: a first strain gauge attached to the strain-measurement strain gauge attaching surface and configured to measure a strain; a second strain gauge provided on the portion other than the strain-measurement strain gauge attaching surface of the flat surface of the cut away part and configured to measure a temperature; and the pipe being connected to a fluid inlet and outlet of the internal space; wherein the pressure gauge is configured to perform a temperature-correction of a pressure value based on the measurement value of the second strain gauge, and the pressure gauge is configured to calculate a pressure value Y by substituting the signal output value X of the first strain gauge into a linear expression Y = aX + b, where a and b are set for each predetermined pressure range, or to calculate a pressure value Y by substituting the signal output value X of the first strain gauge into a polynomial Y = a1X(n-1) + a2X(n-2) +... + anX0 that represents the relationship between the pressure value Y and the signal output value X in all pressure ranges, with ai (i = 1 to n) specified accordingly. a strain-measurement strain gauge attaching surface on the outside surface of the pressure gauge on said flat surface, at a portion having a thickness where pressure produced in the internal space when a fluid is made to flow through the internal space can be substantially detected; and a portion other than the strain-measurement strain gauge attaching surface, the portion having a thickness where the pressure produced in the internal space is substantially impossible to detect; said pressure gauge further comprising: a first strain gauge attached to the strain-measurement strain gauge attaching surface and configured to measure a strain; a second strain gauge provided on the portion other than the strain-measurement strain gauge attaching surface of the flat surface of the cut away part and configured to measure a temperature; and the pipe being connected to a fluid inlet and outlet of the internal space; wherein the pressure gauge is configured to perform a temperature-correction of a pressure value based on the measurement value of the second strain gauge, and the pressure gauge is configured to calculate a pressure value Y by substituting the signal output value X of the first strain gauge into a linear expression Y = aX + b, where a and b are set for each predetermined pressure range, or to calculate a pressure value Y by substituting the signal output value X of the first strain gauge into a polynomial Y = a1X(n-1) + a2X(n-2) +... + anX0 that represents the relationship between the pressure value Y and the signal output value X in all pressure ranges, with ai (i = 1 to n) specified accordingly. a first strain gauge attached to the strain-measurement strain gauge attaching surface and configured to measure a strain; a second strain gauge provided on the portion other than the strain-measurement strain gauge attaching surface of the flat surface of the cut away part and configured to measure a temperature; and the pipe being connected to a fluid inlet and outlet of the internal space; wherein the pressure gauge is configured to perform a temperature-correction of a pressure value based on the measurement value of the second strain gauge, and the pressure gauge is configured to calculate a pressure value Y by substituting the signal output value X of the first strain gauge into a linear expression Y = aX + b, where a and b are set for each predetermined pressure range, or to calculate a pressure value Y by substituting the signal output value X of the first strain gauge into a polynomial Y = a1X(n-1) + a2X(n-2) +... + anX0 that represents the relationship between the pressure value Y and the signal output value X in all pressure ranges, with ai (i = 1 to n) specified accordingly.