We present here a novel microchamber sealing valve that is self-actuated by a pressure change during the temperature change in the thermal activation of reactions. Actuation of our valve requires only the use of the same heating device as employed for the reactions. A thermoplastic UV-curable polymer is used as a device material; the polymer allows realization of the temperature-driven valve actuation as well as the fabrication of multi-layered devices. The self-actuated valve achieves effective sealing of the microchamber for the polymerase chain reaction (PCR) even at 90 °C, which is essential for developing highly parallel PCR array devices without the need for complicated peripherals to control the valve operation.