Ion trapping in a superconducting circular electron storage ring is studied. The electrons are injected at 15 MeV and stored in the ring after acceleration up to 550 MeV. It is shown that the beam size is very large at low energy because of multiple collisions between electrons. This is effective to reduce the tune shift given by the trapped ions in the corresponding energy. Assuming that the species desorbed from the wall under photon irradiation is CO with a yield of 10 -5 molecule/photon and a single set of ion clearing electrodes is installed, the maximum tune shift is 5.5 × 10 -3 in the process of acceleration. This indicates that ion trapping in this ring appears to be harmless after sufficient self-cleaning.