The automatic adjustment method of a microscopic image for automatically adjusting an image on the basis of the lightness of the microscopic image includes distinguishing an observation pixel being an observation target in the image from a non-observation pixel not being an observation target on the basis of the lightness of each pixel of the image, determining a representative value for representing the lightness of the image on the basis of the lightness of a selection pixel identified as the observation pixel and adjusting the lightness of the image on the basis of the representative value.