Initialization of sequential circuits is one of time-consuming processes in test generation for sequential circuits, and hence synthesizing sequential circuits of which synchronizing sequences are short is an important approach to reducing the cost of test generation for the circuits. In this paper, we propose an approach to the synthesis of finite state machines (FSMs) with partial scan. We focus on repeating partial scan for synchronizing FSMs, and present an extended synchronizing sequence which consists of scan inputs and normal inputs, and which takes a circuit to a single specific state, regardless of the initial state. To synthesize synchronizable FSMs, we formulate a problem of minimizing extended synchronizing sequence length, and present a heuristic algorithm for the problem. We show the experimental results of the minimization of extended synchronizing sequence length on MCNC'91 benchmark FSMs. The experimental results show that the proposed heuristic algorithm can find a minimum-length extended synchronizing sequence for most of MCNC'91 benchmark FSMs, and the length of the extended synchronizing sequence is three or less for all the benchmark FSMs.