An acoustic capacity measurement method for finding capacity of a container under test based on a ratio of pressure changes produced in a reference vessel and the container under test by using a speaker to give equal capacity changes to the reference vessel and the container under test and using microphones (6, 7) to measure values of resulting sound pressure changes in the reference vessel and the container under test, wherein the capacity changes vary with time at a frequency with which the speaker is driven; the acoustic capacity measurement method comprising: a pressure change ratio calculating step of calculating a ratio Rf between the measured values of the pressure changes in the reference vessel and the container under test, wherein the pressure change ratio Rf is complex; characterized in that the method further comprises: a rotation step of rotating the pressure change ratio Rf calculated in the pressure change ratio calculating step by an angle of θ found in such a way that the pressure change ratio Rf will vary at a slope of 1 in the complex plane with respect to changes in a surface area of the container under test while the capacity is kept constant, thus calculating a rotated pressure change ratio Ya' having a real part Ya' R and an imaginary part Ya' I, said real and imaginary parts being related to real and imaginary acoustic admittance components Ya R, Ya I of the container under test according to a relationship Ya' R =Ya R /K and Ya' I =Ya I /K respectively, where K is a real number; an oblique coordinate transformation step of transforming said imaginary part Ya' I calculated in the rotation step, into a transformed imaginary component Ya' V = Ya' I - Ya' R in an oblique coordinate system (S, V) whose axes are given by an imaginary axis V and a straight line S with a slope of 1; and a capacity calculating step of calculating the capacity Vt of the container under test by multiplying said transformed imaginary component Ya' V by a predetermined real number Aa, wherein Aa= αK and α is a constant. a pressure change ratio calculating step of calculating a ratio Rf between the measured values of the pressure changes in the reference vessel and the container under test, wherein the pressure change ratio Rf is complex; characterized in that the method further comprises: a rotation step of rotating the pressure change ratio Rf calculated in the pressure change ratio calculating step by an angle of θ found in such a way that the pressure change ratio Rf will vary at a slope of 1 in the complex plane with respect to changes in a surface area of the container under test while the capacity is kept constant, thus calculating a rotated pressure change ratio Ya' having a real part Ya' R and an imaginary part Ya' I, said real and imaginary parts being related to real and imaginary acoustic admittance components Ya R, Ya I of the container under test according to a relationship Ya' R =Ya R /K and Ya' I =Ya I /K respectively, where K is a real number; an oblique coordinate transformation step of transforming said imaginary part Ya' I calculated in the rotation step, into a transformed imaginary component Ya' V = Ya' I - Ya' R in an oblique coordinate system (S, V) whose axes are given by an imaginary axis V and a straight line S with a slope of 1; and a capacity calculating step of calculating the capacity Vt of the container under test by multiplying said transformed imaginary component Ya' V by a predetermined real number Aa, wherein Aa= αK and α is a constant. a rotation step of rotating the pressure change ratio Rf calculated in the pressure change ratio calculating step by an angle of θ found in such a way that the pressure change ratio Rf will vary at a slope of 1 in the complex plane with respect to changes in a surface area of the container under test while the capacity is kept constant, thus calculating a rotated pressure change ratio Ya' having a real part Ya' R and an imaginary part Ya' I, said real and imaginary parts being related to real and imaginary acoustic admittance components Ya R, Ya I of the container under test according to a relationship Ya' R =Ya R /K and Ya' I =Ya I /K respectively, where K is a real number; an oblique coordinate transformation step of transforming said imaginary part Ya' I calculated in the rotation step, into a transformed imaginary component Ya' V = Ya' I - Ya' R in an oblique coordinate system (S, V) whose axes are given by an imaginary axis V and a straight line S with a slope of 1; and a capacity calculating step of calculating the capacity Vt of the container under test by multiplying said transformed imaginary component Ya' V by a predetermined real number Aa, wherein Aa= αK and α is a constant.