A host system defines a plurality of partial areas for a observation object which is a sample, obtains a partial image of an observation object in the partial area captured using a TV camera at the interval of the depth of focus of an objective lens in the depth-of-focus direction of the objective lens, generates from the partial images a focused partial image in which an object contained in the partial area is represented in a focusing state regardless of the difference in a position in the depth-of-focus direction, and generates and displays a focused image of an observation object by combining focused partial images generated in the respective partial area.