also relates to the useful life of the heating resistors fig5 shows the results of the measurement of the cracking characteristics of samples a b and c of conventional heating resistors and sample d of the present invention by a step stress test the applied pulse width was 0 6 m sec the applied pulse cycle was 10 m sec and the step time was 60 seconds in samples a and c the change of the resistance was great and the stress resistance was poor sample b has good stability but the stress resistance was slightly inferior whereas sample d of the present invention had high stability and stress resistance now the boron doped polysilicone will be described which is suitable for use as the heating resistors for the thermal head of the present invention this material is produced by lpcvd method and is a polysilicone layer containing boron at a concentration of from 10 17 to 10 20 cm 3 if the boron concentration is lower than 10 17 cm 3 the resistivity tends to be too high and the desired level of resistance from 200 to 600 u03c9 will not be obtained unless the layer thickness is made thick on the other hand if the boron concentration is higher than 10 20 cm 3 it becomes difficult to obtain a negative temperature coefficient of resistance at a low temperature side within the above mentioned range it is possible to design a heating resistor having any desired level of a temperature coefficient of resistance the boron doped polysilicone layer may be prepared by lpcvd method under such.