Plural lines on a surface of a photosensitive body of an electrophotographic image formation apparatus are simultaneously scanned by plural laser beams for image formation. Each of a pair of semiconductor lasers 11a and 11b biased in an ON state is independently turned on. A surface potential of the photosensitive body in each case is measured by a measurement unit such as a surface potential sensor 30, a surface potential detection circuit 31 and an A/D conversion circuit 32. Based on the measured result, an operational current of each laser is determined by a CPU 33 such that the surface potential of the photosensitive body becomes a target-potential, e.g., 220V to 230V. An image pattern difficult to be influenced by the laser start characteristic and an image pattern easy to be influenced by the laser start characteristic are formed, and the formed image patterns are respectively measured so that the current supplied at no beam emission time is controlled according to the measured results.