A dynamical system with two stable equilibrium points will show a flip-flop motion between the neighborhoods of the two points when it is perturbed by small random noises. A typical example is the stochastic disk dynamo model where the two equilibrium points correspond to the two polarities of the earth's magnetic field. We prove what has been suggested by a computer simulation, that the counting process of the flips or the reversals of the earth's field con-verges to a standard Poisson process if the time is suitably scaled.