Abstract: An anti-thief security sensor assembly wherein the alignment of the respective optical axes of the beam projecting unit and receiving unit with each other can be performed continuously over a required length of time is provided which includes a beam projecting unit (1) for emitting an infrared beam (IR), and a beam receiving unit (2) for receiving the infrared beam (IR) emitted from the beam projecting unit (1). The beam projecting unit (1) includes a first switch (16) adapted to be activated when a cover (10) protecting the beam projecting unit (1) is removed and for outputting a discriminative signal (B) descriptive of removal of such cover, and a transmitting circuit (17) for transmitting the discriminative signal (B) from the beam projecting unit (1) to the beam receiving unit (2), and a recognizing circuit (28) for receiving the discriminative signal (B) and outputting a notifying signal necessary to generate an alarm indicative of the removal of the cover (10).