Poly(butylene terephthalate) (PBT) was depolymerized in excess methanol at high-temperature (473-523 K) and high-pressure (4-14 MPa) conditions. Considering the critical point of methanol (512.6 K, 8.09 MPa), the reaction pressure was varied over the range of 6-14 MPa at the reaction temperature of 513 K. As a result, ca. 20 min was required to recover dimethyl terephthalate and 1,4-butanediol, quantitatively, at any pressure, indicating that the supercritical state of methanol is not a key factor of degradation of PBT and that the effect of pressure is little. On the contrary, when the reaction temperature was varied over the range of 473-523 K at the pressure 12 MPa, the decomposition rate constant of PBT at the reaction temperatures (503-523 K) higher than the melting temperature of PBT (500 K) was much higher than that at 473-483 K. This result indicates that melting of PBT is an important; factor for the short-time depolymerization of PBT.