By infrared absorption spectroscopy we have determined the orientation of the backbone structure of polyimide with alkyl side-chains in the rubbed film. The backbone orientation was determined from the dichroic ratio at normal incidence and the incident angle dependence of absorption by the IR active vibrations polarized along the backbone structure. We found that the backbone structure is oriented along the rubbing direction and tilted up on average by 23 degrees from the surface. The pretilt angle of bulk liquid crystal (LC) in contact with this rubbed film is 9.6 degrees. The average inclination angle of the backbone structure and the pretilt angle are much higher than those for the polyimide without a side-chain reported in our previous papers. This result indicates that the high inclination angle of the backbone structure of polyimide with alkyl side-chains plays an important role in causing a high pretilt angle in bulk LC.