To improve ease of handling by a method wherein a head casing is formed with one end opened to house a heat source part for radiating infrared rays to an opening and a temperature rise recognizing means is attached on the side of the opening.

A head casing 2 is formed with one end thereof opened and a heat source part B is arranged facing an opening thereof to radiate infrared rays. A temperature rise recognizing means E is arranged at the opening part of the head casing 2. In the temperature rise recognizing means E, a thermosensitive discoloring element 51 is buried into a metal plate 50 and covered sealed with a protective seal 52. The thermosensitive discoloring element 51 herein used should be reversible in nature: it changes in color above a specified temperature and returns to the original color when the temperature falls below the value. For example, the temperature of discoloring is set to about 50°C. The rear of the metal plate 50 is colored in a black system to improve the absorptivity of the infrared rays. This enables easy confirmation for the reaching of a treating temperature thereby achieving ease of handling.