The decomposition reaction of pentaerythritol tetranitrate (PETN), C(CH2ONO2)4, under static high pressure was examined by conducting observations and infrared measurements up to 5GPa at around 500K. The thermal decomposition of PETN proceeded rapidly at around 500K and formed a fluid consisting of N2O, CO2, and H2O. The rate of the decomposition reaction at 423K increased as the pressure increased, indicating a negative activation volume. The initial step of the decomposition reaction of PETN under static pressure was presumed to be the elimination of HNO3.