Here we report a patient in whom there was a myopic shift after combined cataract surgery and pars plana vitrectomy against the epiretinal membrane, related to axial measurement estimation error caused by a hidden double-peak appearance on partial coherence interferometry measurement. A 52-year-old female presented with epiretinal membrane and underwent combined cataract surgery and pars plana vitrectomy. Axial length was measured with partial coherence interferometry. Although the signal curve in the summary display showed a single peak, a 1.6 diopter myopic shift occurred. Viewed retrospectively, six of 20 individual signal curves showed a double peak. Most of them showed a higher anterior peak, with only one having a higher posterior peak. The other 14 curves showed a single peak at a similar distance to an anterior peak. The anterior peak appeared to be derived from the epiretinal membrane. The possibility of a double peak should always be considered in patients with epiretinal membrane even if the summary display of the partial coherence interferometry measurement shows a single peak. Checking all signal curves would reduce the risk of missing a hidden double peak.