To provide a fire sensor high in reliability which finds a fire rapidly spreading like bursting into flames in its early stage.

The fire sensor comprises: a temperature detecting means 2 which detects a temperature T and a temperature rise ΔT; a smoke density detecting means 3 which detects smoke density S; a storage means 4 which stores a threshold curve of a two-dimensional coordinate system regarding the smoke density S; an arithmetic means 10 which computes coordinate positions from detection signals inputted at every specified time from respective detecting means and decides whether a coordinate position exceeds the threshold curve in the increasing direction of the smoke density S; a counting means 11 which counts the continuation of a state wherein the coordinate position exceeds the threshold curve; a fire decision means 12 which outputs a fire signal when the count value exceeds a specified decision value; and a decision value varying means 13 which sets the specified decision value smaller when the temperature rise ΔT or temperature T exceeds the specified threshold before the count value exceeds the specified threshold.