There are some results indicating that a quantum computer seems to be more powerful than ordinary computers. In fact, P.W. Shor showed that a quantum computer can find discrete logarithms and factor integers in polynomial time with bounded error probability. No polynomial time algorithms to find them using ordinary computers are known. In this paper, we show that the cycles in some kinds of periodic functions, e.g., functions proposed as pseudo-random generators, can be found in polynomial time with bounded error probability on a quantum Turing machine. In general, it is known that ordinary computers take exponential time to find the cycles in periodic functions.