To provide a method for adjusting lightness of an image in an imaging type microscope device capable of rapidly adjusting lightness of an image even when brightness is changed according to an object to be observed.

A user sets the diameter of a pinhole 122 of an observation optical system to the maximum by using a low-powered objective 118b, so as to acquire a microscopic image, first. When the user switches the observation optical system in order to narrow down an observation surface after completing adjustment of brightness, the acquired microscopic image is stored as reference information showing the brightness of the image that is reference. An automatic adjusting means acquires the microscopic image after completing switching of the objective or the observation optical system, reads out the stored reference information on the brightness, compares it with the acquired information on brightness of the image, calculates a gain value of a photodetector 123, and sets the obtained gain value to the photodetector 123, thereby adjusting the brightness of the image after switching. The above processing is repeated until deviation from the reference information showing the brightness of the image is eliminated.