PURPOSE: To provide a beam deflecting device capable of emitting a parallel beam with a simple construction and a defect inspection device using this beam deflecting device.

CONSTITUTION: This beam deflecting device 4 is constituted of optical material, is provided with a rotating prism body 5 having more than two pairs of parallel optical planes placed opposite to each other, a rotation driving device rotating the rotating prism body 5 around a rotation axial line R parallel to an optical plane. In the device, a light beam made incident along an optical axis L heading to the rotation axial line in a plane perpendicular to the optical plane and the rotation axial line is emitted as a light beam which is parallel to an optical axis and whose distance from the optical axis is changed with time by rotating the rotating prism body 5 around the rotation axial line R. The device is extremely useful for inspecting defects on a sample in which many elements are arranged regularly since the surface of the sample can be scanned with a vertical incident beam by using the beam deflecting device.