To provide an error correction method for a density measurement error for correcting the measurement error caused due to a difference in an optical path length inside a sample in a transmission density measurement by a two-dimensional imaging device.

In the method, the density measurement error in the center and a peripheral part of a screen is corrected when the transmission density measurement or a transmittance photographing operation is performed by using the two-dimensional imaging device. In the error correction method, a reference material for density correction having a uniform thickness and a uniform density is measured two-dimensionally, its measured data is converted into a density, a factor by which a density value in all pixels becomes a density value in the center of the screen is found for each pixel, the value is used as correction data so as to be multiplied by a density value of photographed data on an arbitrary sample, and the density measurement error is corrected.