A blood flow index calculating method, wherein a computer executes: obtaining, by a camera, time-series of images capturing a first site of a living body and a second site of the living body, the first site being a part of the living body of a subject, and the second site being a site different from the first site; extracting an area of the first site of the living body and an area of the second site of the living body, extracted areas being included in each of the images; detecting a pulse wave pattern of the first site of the living body from the area of the first site of the living body and detecting a pulse wave pattern of the second site of the living body from the area of the second site of the living body; calculating a delay amount from the pulse wave pattern of the first site of the living body and the pulse wave pattern of the second site of the living body; and calculating an index related to blood flow by using the delay amount, wherein as the detecting of the pulse wave pattern of the first site of the living body, a wavelength component of a particular frequency band (Rn, Gn, Bn) other than a frequency band that pulse waves are possibly in among respective wavelength components is extracted, for each of the wavelength components (R, G, B), a correction coefficient (k) is calculated by dividing an absolute value of a strength of the first wavelength component of the particular frequency band (Rn, Gn, Bn) by an absolute value of a strength of the second wavelength component of the particular frequency band(Rn, Gn, Bn), at least one signal of the signals (Rs, Gs, Bs) of the representative values of the respective wavelength components is multiplied by the correction coefficient(k), the representative value being an average, median or mode value of pixel values of respective pixels included in the area, and a waveform of a signal in which the wavelength components of the particular frequency band have been offset with one another is detected by calculation of the difference between the signals of the representative values of the respective wavelength components after the multiplication by the correction coefficient. obtaining, by a camera, time-series of images capturing a first site of a living body and a second site of the living body, the first site being a part of the living body of a subject, and the second site being a site different from the first site; extracting an area of the first site of the living body and an area of the second site of the living body, extracted areas being included in each of the images; detecting a pulse wave pattern of the first site of the living body from the area of the first site of the living body and detecting a pulse wave pattern of the second site of the living body from the area of the second site of the living body; calculating a delay amount from the pulse wave pattern of the first site of the living body and the pulse wave pattern of the second site of the living body; and calculating an index related to blood flow by using the delay amount, wherein as the detecting of the pulse wave pattern of the first site of the living body, a wavelength component of a particular frequency band (Rn, Gn, Bn) other than a frequency band that pulse waves are possibly in among respective wavelength components is extracted, for each of the wavelength components (R, G, B), a correction coefficient (k) is calculated by dividing an absolute value of a strength of the first wavelength component of the particular frequency band (Rn, Gn, Bn) by an absolute value of a strength of the second wavelength component of the particular frequency band(Rn, Gn, Bn), at least one signal of the signals (Rs, Gs, Bs) of the representative values of the respective wavelength components is multiplied by the correction coefficient(k), the representative value being an average, median or mode value of pixel values of respective pixels included in the area, and a waveform of a signal in which the wavelength components of the particular frequency band have been offset with one another is detected by calculation of the difference between the signals of the representative values of the respective wavelength components after the multiplication by the correction coefficient.