We investigate the spin evolution of the binary X-ray pulsar 2S 0114+650, which possesses the slowest known spin period of ～2.7 hours. We argue that, to interpret such long spin period, the magnetic field strength of this pulsar must be initially >～10 14 G, that is, it was born as a magnetar. Since the pulsar currently has a normal magnetic field ～1012 G, our results present support for magnetic field decay predicted by the magnetar model.