A focusing method includes a step of preparing a microscope, a step of mounting a sample, and a predetermined processing step, the predetermined processing step includes a step of receiving light emitted from the observation optical system, a step of obtaining the quantity of light based on light from a predetermined region of the received light, a step of calculating a difference or a ratio between the quantity of light in the predetermined region and the quantity of light as a reference, a step of comparing a calculation result with a threshold, and a step of changing the distance between the sample and the observation optical system, and in the step of preparing, a partial region of illumination light is shielded or darkened, and when the result of the calculation is equal to or smaller than the threshold, the predetermined processing step is terminated.