1. A defect inspection method, comprising: driving a stage or an objective lens facing an observation object in order to change an observation part of the observation object placed on the stage to a reference part determined to be normal beforehand within the observation object; performing focusing control with a focusing control unit which changes a relative distance between the stage and the objective lens in a direction of the optical axis of the objective lens so that automatic focusing is achieved on the reference part according to a first focusing control parameter; determining a second focusing control parameter based on sample information obtained when performing the focusing control to achieve the automatic focusing on the reference part using the first control parameter, wherein the sample information contains at least one of information about the focusing position of the reference part and information about a light amount according to light reflected from the reference part, and the second focusing control parameter contains at least one of movement speed used by said focusing control unit, search range used when acquiring the observation object, autofocus method, offset amount, and contrast threshold; performing focusing control so that focusing is achieved on the reference part according to the first focusing control parameter; obtaining a pattern image of the reference part; driving the stage or the objective lens in order to change the observation part of the observation object to a part to be inspected, which becomes a target of inspecting for the presence or absence of a defect within the observation object; performing the focusing control with the focusing control unit so that automatic focusing is achieved on the part to be inspected according to the second focusing control parameter; then obtaining a pattern image of the part to be inspected; and detecting the presence or absence of an abnormal condition of the part to be inspected by making a comparison between the pattern image of the reference part and the pattern image of the part to be inspected.