devices 20 a to 20 e in the storage section 20 stores the transmitted light signals l 1 to l 5 as shown in fig2 the processor 30 comprises a segmented data storage element 32 including storage elements 32 a to 32 e which divide the transmitted light signals l 1 to l 5 having been input from the temporary storage devices 20 a to 20 e into data pieces corresponding to segments of a predetermined time period e g 0 5 second and sequentially store the segmented data pieces as time series data the processor 30 further comprises a first calculator 34 including calculator elements 34 a 34 b 34 c and 34 d which respectively calculate gradients u03c6 12 u03c6 32 u03c6 42 and u03c6 52 of regression lines with regard to the transmitted light signals l 1 to l 5 having been stored in the segmented data storage element 32 as the segmented data pieces the processor 30 further comprises a second calculator 36 which obtains solutions to simultaneous equations with regard to the gradients u03c6 12 u03c6 32 u03c6 42 and u03c6 52 of the regression lines having been obtained by the first calculator 34 and a third calculator 38 which smoothes the solutions of the simultaneous equations thereby obtaining an oxygen saturation in blood spo 2 meanwhile the solutions may be determined after the gradients have been smoothed next processing operations for calculating an arterial oxygen saturation performed by apparatus having the configuration.