Electron spin resonance (ESR) dating of a boring core down to the depth of 800 m at Hachobara Geothermal Power Station has been made utilizing signals of defects produced by natural radiation. The lifetime and the growth of a defect at a geothermal temperature are estimated theoretically from the annealing temperature. Tentative ESR ages at several depths using signals at g=2.0060 and g=2.0010 presumably due to E1 and E' centers are interpreted taking the lifetimes at the temperature into account. Organic methanol found by irradiation of cores may be condensed by natural distillation in a geothermal field.