An infrared detector includes a circuit block carrying an infrared sensor element and electronic components. The circuit block is composed of a dielectric resin layer and a first substrate formed with a circuit pattern and mounting the electronic components. The dielectric resin layer is formed in its top with a recess which defines around its periphery with a shoulder for supporting opposite ends of the infrared sensor. The first substrate is integrated to the lower end of the dielectric resin layer with at least one of the electronic components being molded into the dielectric resin layer to make the circuit block of a unified mold structure. Thus, a part or all of the electronic components are molded into the dielectric layer to realize the circuit block of a simple and low profile structure, while retaining an advantage of keeping the infrared sensor element sufficiently away from the electronic components and an associated electronic circuit, thereby assuring to give the infrared detector which is simple in construction, economical in cost, and reliable in the infrared detection.