A thermal trip device in which a bimetal (2) is heated by overcurrent and performs trip operation of a circuit by curvature of the heated bimetal (2), wherein at least one part of the surface of the bimetal (2) is made to be black or matte black (7). Thereby, temperature of the bimetal (2) can be highly accurately measured using a no-contact thermometer. Furthermore, a temperature measurement part (8) of the bimetal is provided with a bending part (11), and the surface of the bending part is made to be matte black.