The molecular displacements of the E --> C and B --> C phase transitions of stearic acid have been investigated by an infrared oblique transmission method. This enables us to determine unambiguously the three-dimensional arrangement of functional groups from intensity changes associated with the variation of two setting angles of the sample measured against the polarized incident radiation. In particular, the structural changes occurring in single-crystal specimens of double-layered polytype (Orth II) were investigated. It has been clarified that polymorphic-polytypic composite structural changes occur for the E(Orth II) single crystal; i.e., the double-layered polytype of E transforms to the single-layered one of C through two different precessional molecular displacements occurring alternately along the stacking direction of bimolecular layers. Similar structural changes took place during the B(Orth II) --> C phase transition.