An infrared sensor (20) is held in a housing (10) by a sensor holder (28) made of synthetic resin. A waveguide (30) for guiding the infrared rays emitted from an object to the infrared sensor (20) is supported by waveguide holders (31, 32) made of synthetic resin (or metal). There is an air layer (heat-insulating layer, or clearance) (34) between the inner end (distal end) (30b) of the waveguide (30) and the infrared sensor (20). A probe (40) made of synthetic resin is installed that surrounds the portion of the waveguide (30) projecting outside. There is also an air layer (heat-insulating layer, or clearance) (33) between the front end of the probe (40) and the front end (outer end) (30a) of the waveguide (30). A cap (50) is fitted on the front end of the waveguide (30).
