A continuous-thrust guidance algorithm suitable for a formation flying of a large number of spacecraft having electric propulsion systems are discussed. This algorithm generates continuous-thrust transfer trajectory between any two arbitrary states, which can be optimized in term of total fuel consumption for a formation changing. The algorithms are effective not only in term of fuel consumption but also in term of information quantity to be shared among member spacecraft. Moreover taking computational simplicity into consideration, a quasi-optimal continuous-thrust guidance algorithm are derived.