A non-volatile flip-flop (NVFF) introducing MTJ has many strong points in high endurance and read/write performance, and hence is very attractive as a component to be used for power gating of sequential circuits. However, large write-energy to MTJ becomes a big obstacle in achieving low energy dissipation. This paper proposes a NVFF circuit enabling to verify the success of a store operation to MTJ and retry it by prolonging the store time. We designed a NVFF circuit with this feature and applied it to 20,000 flip-flops in a dynamically reconfigurable processor (DRP). We conducted simulations considering write time variations caused by various factors such as process variations and thermal fluctuations. The results demonstrated that the proposed approach reduces store energy by 35-36% at four image-processing applications and the break-even time (BET) for non-volatile power gating is 2.0-2.9us at the 0.004% write error rate, at which no failures occur for the total number of NVFFs in the DRP.