A temperature sensitive piece 9 has a waxed layer 7 formed on a colored face 4 of a colored paper 6 having an arbitrary plane shape. The waxed layer 7 is formed of a mixture of a proper amount of viscous material and powdered petroleum wax melting at selectively preset temperature. The temperature sensitive piece 9 is affixed to a sheet-shaped base 1 with the waxed layer 7 being located close to the front face of the temperature sensor label. The entire face of the base 1 facing the front face of the temperature sensor label is covered with a transparent film 5.