One technological problem of rubber O-rings used in hydrogen equipment is the occurrence of internal fractures called blisters caused by gas decompression. The use of damaged rubber leads to the decrease in sealability. Our previous study clarified influencing factors on such rubber fractures by quality engineering, based on durability tests with a few hundred pressure cycles. However, the seal durability should be evaluated under the long-term pressure cycle condition which reproduces a realistic pressure environment. This study determined a realistic pressure cycle condition influencing the fracture and sealability by using a developed durability tester. Under the condition, precise durability tests at 5,500 cycles were performed. The results showed that no leakage due to fatal fractures occurred at the condition.