PURPOSE: To control the gray scale accurately and to improve record quality by calculating the total number of heating elements to be conducted concurrently and controlling the duration of single conduction so that a substantially constant power is applied on the heating element through single conduction.
CONSTITUTION: A thermal head 2 in the thermal transfer recorder comprises a plurality of heating elements and the total applying power thereof is controlled individually with conduction times corresponding to the gray scale according to the dots which determine the number of pixels of a recording image. Furthermore, an ON dot number calculating circuit 22 for calculating the total number of heating elements to be conducted concurrently, i.e., the number of ON dots (n), is provided and connected with a gradation counter 10 and a line buffer 7. Output from the calculating circuit 22 is fed to a power control means, i.e., a strobe signal generating circuit 23, in order to control the duration of strobe signal variably. Duration of single conduction is controlled so that the power to be applied on the heating elements through single conduction will be constant regardless of the total number of the heating elements to be conducted concurrently.