A pulse oximetry includes: irradiating living tissue with a plurality of light beams of different wavelengths； receiving the light beams transmitted through or reflected from the living tissue and converting the received light beams to electric signals which correspond to the different wavelengths； time-segmenting time series data of the electric signals； calculating, with respect to each of the segmented time series data of the electric signals, a slope value of a regression line between each two of the electric signals； calculating SaO2 based on the slope value of each of the segmented time series data of the electric signals； constructing a histogram of SaO2 for each predetermined number of time segments； and obtaining a mode value from the histogram as SpO2 to be output of the pulse oximetry.
