To provide an infrared detector in which its responsiveness to outdoor temperature variation can be improved while its constitution is simplified.

In the infrared detector 300 formed by assembling a cap 220 with an entrance window section 221 for transmission of infrared ray to a pedestal 210 so that the infrared sensor 100 is housed in a state where the infrared sensor 100 equipped with a detecting element 20, a part of which is, at least, formed on a membrane 13 of the substrate 10, generating electric signals induced by the temperature variations at the reception of infrared ray, and an infrared absorption film 30 formed on the membrane 13 so as to cover, at least, a part of the detecting element 20 is arranged on the pedestal 210, a protuberance 222 which is projected toward the infrared sensor 100 from the cap 220 so as to prevent no incoming infrared ray into an infrared absorption film 30 through the entrance window section 221 and produced by material of greater thermal conductivity than gas enclosed in a case 200 is united with the cap 220.