A substrate has M×N heater elements divided into N blocks and time divisionally driven in units of the block, and signal lines for supplying M bits of signals for selecting elements to be driven in each block. The signal lines are divided into two groups, each extending toward the center of a heater element array from a start point at either end side of the array in the array direction, and being used to select M/2 elements closest to the end side of the array. In each groups, the M/2 bit signal lines are arranged so that the number of the signal lines is decremented as the distance from the start point increases, and that the remaining signal lines are shifted toward the array. A temperature sensor for the substrate is provided in an empty area produced at a position corresponding to the center of the array by the shift.