Electrons or positrons were stored in the electron storage ring of the Photon Factory (PF). The vacuum of the PF was so good that differences in the instantaneous beam lifetimes between electron and positron storage modes were very slight. Abnormal decrements of the stored beam current observed in the electron storage mode did not appear in the positron storage mode. Thus, the decay curves of the beam current in the positron storage mode were smoother than those in the electron storage mode. Though the instantaneous beam lifetime was nearly the same in storage modes, the practical beam lifetime of a positron was about 10% longer than that of an electron.

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