We examined whether people can retain an implicit memory for unfamiliar faces over several weeks by using an indirect recognition procedure (Terasawa & Ohta, 1993). The procedure was composed of two sessions with a long-term interval. Two experiments using different intervals ─ 19 days (Experiment 1) and 7 weeks (Experiment 2) ─ were conducted. In each session, participants were presented with line drawings of faces and were asked to rate their preferences. The number of presentations varied for each condition in the first session. Participants were also asked to perform an unexpected recognition test for the faces presented after the rating task in the second session. Results from both experiments showed that the rates of correct and false recognition of faces increased significantly with the number of presentations in the first session. The findings indicate that long-lasting implicit memory can be formed for unfamiliar faces by incidental learning.