A pyroelectric infrared ray detector of the so-called dual structure (10, 20) for detecting an intruder or the like through differential output of two pyroelectric infrared ray detecting elements (10a, 10b, 20a, 20b) connected in parallel or series to each other. The infrared ray detector comprises a pair of pyroelectric infrared ray detecting elements (10a, 10b, 20a, 20b) having substantially identically directed light receiving surfaces and electrically connected to each other and a shield member (12, 21, 29, 63) arranged in front of the said light receiving surfaces to partially shield the infrared ray detecting elements against incidence of infrared light. The shield member is arranged in a plane extending between the infrared ray detecting elements to separate the same on both sides thereof.