To provide an enzyme activity value measuring device using a spectrophotometer in which the best time section can be regularly selected to precisely calculate the activity value.

A window setting part 22 sets a window having a minimum time width designated by a measuring person or larger than it in an optional position of the time change curve of absorbance, a correlation coefficient arithmetic part 23 calculates the correlation coefficient of the data contained in the window. A window selecting part 24 compares all the correlation coefficients when the time width and set position of the window are changed and selects a time section having the best linearity of absorbance curve, and an activity value arithmetic part 25 calculates the activity value from the gradient within this time section.