1. A method of detecting a defect by inspecting a specimen including the steps of: illuminating a surface of a specimen on which plural patterns are formed with an elongated shape light flux from a first elevation angle, said specimen being set on a stage which is continuously moving in one direction; capturing an image of the specimen illuminated by the elongated shape light flux with a linear image sensor of an optical detecting unit installed in a second elevation angle direction; and detecting a defect on the specimen by processing a signal output from the linear image sensor, wherein a longitudinal direction of said elongated shape light flux is perpendicular to a direction along which said stage is continuously moving, and wherein the stage moves within a plane defined by two axes of a Cartesian coordinate system and the elongated shape light flux is directed along a third axis of the Cartesian coordinate system.