A polytype phenomenon, i.e., a polymorph of layer stacking structure, has been discovered for a side-chain-type ferroelectric liquid-crystalline polymer. The samples cooled from the isotropic state to room temperature at only slightly different rates (0.2 and 0.1-degrees-C/min, for example) showed quite different X-ray diffraction patterns at low Bragg angle, from which the repeating period of layers constructed by mesogenic side groups was evaluated as 68.4 angstrom for the Sl phase obtained at the cooling rate 0.2-degrees-C/min and 136.8 angstrom for the S2 phase at 0.1-degrees-C/min. The wide-angle X-ray diffraction and infrared/Raman spectral data were almost the same between these two samples, indicating they take essentially the same inner-layer packing structure. That is, these two samples are different only in their layer stacking structure, i.e., a phenomenon of polytype. The S1 and S2 phases are speculated to be single- and double-layered structures, respectively, where the one layer or the two layers make a repeating period along the normal to the sample plane. A possibility of ferroelectric and antiferroelectric structures has been discussed for the Sl and S2 phases, respectively. By measuring the temperature dependence of the X-ray diffraction, a complicated phase transition has been found to occur among many types of phase with various repeating periods of layers : S1(68.4 angstrom) 80-degrees-C --> S4(62.8 angstrom) and/or S3(218.4 angstrom) 90-degrees-C --> S(C)*(64.0 angstrom) 100-140-degrees-C --> S(A)(68.8 angstrom) 155-degrees-C --> Ch 165-degrees-C --> isotropic where S3 is considered to be a triple-layered phase and S(C)*, S(A), and Ch are smectic C, smectic A, and cholesteric phases, respectively.