An infrared ray radiated from a region of a surface of an object to which a coating film (20) of a coating material is provided is detected by an infrared sensor (42). The coating film (20) includes a porous ceramic particle (22) and a binder (24), and the ceramic particle (22) includes a compound represented by a compositional formula of any of AaRbAlcO4, AaRbGacO4, Rx, AlyO12, and RxGayO12. Here, A is one or more elements selected from a group consisting of Ca, Sr, and Ba, and R is one or more elements selected from a group consisting of rare earth elements. Also, a is equal to or greater than 0.9 and equal to or less than 1.1, b is equal to or greater than 0.9 and equal to or less than 1.1, c is equal to or greater than 0.9 and equal to or less than 1.1, x is equal to or greater than 2.9 and equal to or less than 3.1, and y is equal to or greater than 4.9 and equal to or less than 5.1. A porosity of the ceramic particle (22) is equal to or greater than 20% and equal to or less than 40%.