A temperature sensor forming method and forming die are provided which perform insert-forming of a temperature sensor with a minimum of processes and in a short time. The temperature sensor formed has a temperature sensing element 1 arranged in a predetermined position inside a resin case 2. The process does not leave traces of pins that would allow water to penetrate the interior of the case. The process comprises a first step of supporting the temperature sensing element 1 with first and second slide blocks 6c and 6g. The process comprises a second step of injecting a molten forming resin 2a into a forming die 6 and retracting the first slide block 6c by injection pressure while the temperature sensing element 1 is being supported by the second slide block 6g. The process comprises a third step of retracting the second slide block 6g while injecting the molten forming resin 2a into the forming die 6. The receiving area S1 of the first slide block 6c is larger than the receiving area S2 of the second slide block 6g.