1. A defect inspection apparatus, comprising: an observation part changing unit changing an observation part of an observation object by driving a stage on which the observation object is placed, or an objective lens as opposed to the observation object; a focus direction driving unit driving at least one of the stage and the objective lens in order to achieve focus on the observation object placed on the stage; a focusing controlling unit performing focusing control by making said focus direction driving unit drive at least one of the stage and the objective lens in order to achieve focus on the observation object; a focusing control parameter setting unit setting focusing control parameters used for the focusing control performed by said focusing controlling unit; a pattern image obtaining unit obtaining a pattern image of a predetermined part by making said observation part changing unit drive the stage or the objective lens in order to change the observation part of the observation object to the predetermined part within the observation object, and by making said focusing controlling unit perform the focusing control according to the focusing control parameters set by said focusing control parameter setting unit in order to achieve focus on the predetermined part; a pattern image storing unit storing the pattern image obtained by said pattern image obtaining unit; and a detecting unit detecting presence/absence of an abnormal condition of a part to be inspected by making a comparison between a pattern image, which is stored in said pattern image storing unit and obtained by said pattern image obtaining unit, of a reference part determined to be normal beforehand within the observation object, and a pattern image, which is obtained by said pattern image obtaining unit, of the part to be inspected, which becomes a target of inspecting presence/absence of a defect within the observation object, wherein the focusing control parameters, which are used for the focusing control performed when said pattern image obtaining unit obtains the pattern image of the part to be inspected, are determined based on sample information obtained by the focusing control performed when said pattern image obtaining unit obtains the pattern image of the reference part.