In the thermosensor of the invention, both ends 21, 22 of an elastic member 2 are fixed to a body 1 in a state where the elastic member 2 is compressed in a longitudinal direction, to form the elastic member 2 into a convex curved shape, one end side of the convex curved shape is raised by a predetermined angle .theta.L' with respect to the body 1, a flexure angle of another end 22 of the convex curved shape is zero, the fixation of one end portion 21 of the elastic member 2 and the body 1 is conducted via a fusible material 3, and a melting point or a softening point of the fusible material 3 is an operating temperature.