At each of mutually different multiple focal positions, focal adjustment parameter values are obtained from images of standard particles made of the same substance. Each focal adjustment parameter value is figured out as any one of the ratio between the density value around the center of the standard particle image and the density value around the outline, the difference therebetween, and the density value around the center. The in-focus position is adjusted on the basis of the relationship between the obtained focal adjustment parameter values and the focal positions. Moreover, on the basis of the relationship between the focal adjustment parameter values and the focal positions, the parameter values are converted into focal positions, and the focal positions and dispersion thereof are used to check the displacement of the in-focus position and the thickness of the sample liquid.