A thermal displacement element comprises a substrate, and a supported member supported on the substrate. The supported member includes first and second displacement portions, a heat separating portion exhibiting a high thermal resistance and a radiation absorbing portion receiving the radiation and converting it into heat. Each of the first and second displacement portions has at least two layers of different materials having different expansion coefficients and stacked on each other. The first displacement portion is mechanically continuous to the substrate without through the heat separating portion. The radiation absorbing portion and the second displacement portion are mechanically continuous to the substrate through the heat separating portion and the first displacement portion. The second displacement portion is thermally connected to the radiation absorbing portion. A radiation detecting device comprises a thermal displacement element and a displacement reading member fixed to the second displacement portion of the thermal displacement element and used for obtaining a predetermined change corresponding to a displacement in the second displacement portion.