A rotor of an alternating current generator has a field excitation assembly supported on a rotor shaft including a pair of rotor segments forming a cavity therein, a bobbin made of an insulating resin disposed in the cavity and a field excitation coil wound on the bobbin. The bobbin has a cylindrical portion and a pair of flanged portions at both ends thereof for forming room for the field excitation coil. On one of the flanged portion, a pair of projections are integrally formed therewith extending axially and outwardly, which are press-fitted into a pair of through holes formed on one of the rotor segments. Each of the projections is formed with not only an axially extending groove for receiving a field coil terminal, but a radially and axially extending slit communicating the outside of the projection with the groove for guiding and receiving a coil starting or terminating end of the field coil. The coil ends are hung on the field coil terminals press-fitted into the projections, whereby neither the coil starting nor terminating end is drawn back out of the bobbin even before the bobbin is assembled in the rotor segments.