A spectrophotometer which comprises a monochromator, a pair of cells to which the light from the monochromator is projected, means for converting the light from the cells into corresponding electrical signals, means for comparing the electrical signals to produce an output, means for magnetically recording the output from the comparing means, means connected to the output of the comparing means to perform predetermined arithmetic operations thereon, and means for recording the output from the last-mentioned means, whereby various types of spectra of substances such as derivative spectra, difference spectra and the derivatives thereof can easily be obtained.