In single‐flux quantum (SFQ) memory cell operation, a stochastic phenomenon occurs in certain ranges of bias current. These regions, in which the memory cell stochastically switches to the voltage state or to the other vortex mode, are called the gray zone. The gray zone reduces the operating margin of the cell, because the bias current region in the gray zone must be avoided for memory operation. It is very important to investigate the origin of the gray zone for the design of the cell. But the gray zone has not ever been discussed in detail. We have studied memory operation from the viewpoint of the thermal fluctuation of the potential energy of the cell. Our conclusions are as follows. (1) The gray zone originates in bias current fluctuations, caused by the thermal fluctuation of the potential energy of the cell. (2) In the gray zone the probability of transition to the voltage state does not necessarily increase monotonically with the bias current. This is because the transition to the voltage state occurs in several bias current regions, even if there is no thermal fluctuation, for a memory cell with the large McCumber factor.