To provide a heat sensor capable of immediately detecting that an external temperature becomes a prescribed temperature or above, and to provide a fire alarm using it.

This heat sensor has: a tube body 101 provided with a lid part 101a on one end side; and a detection signal output part 102 provided in a cavity part of the tube body 101, outputting a detection signal when a contact 102a with the lid part 101a turns off based on thermal expansion of the tube body 101 when the external temperature becomes the prescribed temperature (e.g. 60C) or above. Thereby, external air heat can be directly conducted to the tube body 101, and the thermal expansion or contraction of the tube body 101 accompanying a change of the external temperature can be rapidly performed. Accordingly, it can be immediately detected that the external temperature becomes the prescribed temperature or above, and detection accuracy of indoor occurrence of a fire, or the like, for example, can be improved.