We have proposed a new method of electron spectroscopy which is measuring charge transfer (CT) force by atomic force microscopy. We have applied this new method to a typical molecularly doped polymer system [N,N'-diphenyl-N,N'-bis(3-methylphenyl)-[1,1'-biphenyl]-4,4' diamine (TPD) and polycarbonate] to elucidate the electronic structure of the system. CT force spectra obtained show that the energy gap (E-g) continuously decreases with increasing the TPD concentration. We discuss the change in E-g from two different points of view: homogeneous and inhomogeneous distributions of TPD. (C) 2001 American Institute of Physics.