A method for correcting an instrumental error due to wavelength error of a spectroscope of an optical analyzer for measuring an optical density reflected from or transmitted through a dry analysis element. One standard color plate having a standard optical density OD.sub.ST is measured by an optical analyzer to be corrected to obtain an measured value OD.sub.M. A measured optical density OD.sub.S of the element applied with a sample through the optical analyzer to be corrected is corrected by using a ratio of OD.sub.M /OD.sub.ST to obtain a corrected measured value OD.sub.C of the sample. The standard color plate to be used contains an indicator dye contained in the non-reacted dry analysis element or a dye having an absorption spectrum same as or similar to that of the indicator dye in the measurement wavelength range. Another correction method in consideration of a layer coefficient of a dry analysis element is also provided.