To overcome the drawbacks of the transfer Hamiltonian method of Bardeen, which is predominantly used in the theory of the scanning tunneling microscope (STM), we develop a new system of formulation in three dimensions accounting for the explicit correlation between the measured current and the electronic states of both the tip and the sample surfaces with the coupling between them being taken into account. It is found that the contrast or the corrugation of the STM image can be essentially modified by the nature of tip electronic state and that the resolution of the STM is determined by the coupling between the tip and the sample along with the tip electronic state. Comparison with other theories are given.