The crystal structure of the orthorhombic polyoxymethylene (POM) has been refined by analyzing the synchrotron X-ray diffraction data measured for a micrometer-size single crystal of moth-type morphology. The 3-dimensional elastic and compliance constants tensors were calculated for both the trigonal and orthorhombic phases on the basis of the thus-obtained accurate structure information. The crystal structure information allowed us also to discuss the solid-state phase transition behavior from the orthorhombic to the trigonal phase occurring at around 78 degrees C. For this purpose, we have performed the temperature-dependent synchrotron X-ray scattering measurement for a tiny moth-type single crystal and revealed the geometrical relation between these two crystals. The free energy was calculated as a function of temperature for these two accurately-analyzed crystal forms using a density functional theory (DFT). The result showed that the orthorhombic phase is thermodynamically more stable than the trigonal phase in a low temperature region, and the stability was reversed at about 225 K. This temperature corresponds to the phase transition point from the orthorhombic to trigonal phase, which is however considerably lower compared with the actually-observed transition point. The large discrepancy might be ascribed to the additional contribution of the entropy term due to the structural disorder and/or the thermal motion of chains with a large amplitude, which was not taken into account in the DFT calculation.