The present invention relates to a differentiation marker and a differentiation controlling technique for an eye cell. More particularly, the present invention has attained an object of providing a differentiation marker for an eye cell among the aforementioned problems, by providing a marker for identifying a cell having a high proliferation ability among corneal endothelial cells and/or the differentiation ability of a corneal endothelial cell, the marker comprising GPR49/LGR5, as well as a detection agent or detection method for identifying a cell having a high proliferation ability among corneal endothelial cells and/or the differentiation ability of a corneal endothelial cell, comprising a substance binding to GPR49/LGR5. In addition, the present invention has attained an object of providing a differentiation controlling technique for an eye cell, by providing an agent for suppressing differentiation and/or promoting proliferation of an eye cell, comprising R-spondins.