An auto-program voltage generator in a nonvolatile semiconductor memory having a plurality of floating gate type memory cells, program circuit for programming selected memory cells, and program verification circuit for verifying whether or not the selected memory cells are successfully programmed comprises a high voltage generator for generating a program voltage, a trimming circuit for detecting the level of the program voltage to increase sequentially the program voltage within a predetermined voltage range every time the selected memory cells are not successfully programmed, a comparing circuit for comparing the detected voltage level with a reference voltage and then generating a comparing signal, and a high voltage generation control circuit for activating the high voltage generator in response to the comparing signal.