Tumor cell plasticity and tumor heterogeneity are involved in therapy resistance. Cancer stem cells (CSCs) refer to tumor cells that have the ability to self-renew, and generate the diverse cells that comprise the tumor and complicate tumor heterogeneity. In recent years, CSCs have been reported to emerge from non-CSCs, which is known as tumor cell plasticity; however, the mechanism has not been fully elucidated. The present study investigated tumor cell plasticity from the viewpoint of aldehyde dehydrogenase 1 family member A1 (ALDH1A1) activity, which is one of the markers of CSCs. In the endometrioid carcinoma cell line HEC-1B, the ALDH1A1-low population spontaneously yielded an ALDH1A1-high population, mimicking tumor cell plasticity, and it was revealed that the mixture of the ALDH1A1-high population with the ALDH1A1-low population sometimes accelerated the transition from an ALDH1A1-low to ALDH1A1-high population. Two distinct HEC-1B sublines were established. One of the two sublines accelerated such a transition and the other did not show such acceleration. In the former subline, the effect of the ALDH1A1-high population was abolished when the direct cell-cell contact between ALDH1A1-high and ALDH1A1-low populations was inhibited. By comparing the two sublines, the neuronal membrane glycoprotein M6-b (GPM6B) was identified as the candidate mediating tumor cell plasticity. GPM6B was expressed in the border of ALDH1A1-expressing tumor cells and non-expressing tumor cells in clinical samples of EC. Notably, knockout of GPM6B decreased ALDH1A1 expression, whereas its overexpression increased the expression of ALDH1A1, suggesting that GPM6B mediated the induction of ALDH1A1 and the plasticity of CSCs.