Provided is a method which is capable of determining, at an initial stage of differentiation, the differentiated state of undifferentiated stem cells. The present invention relates to "a method for determining a differentiated state of a cell comprising detecting expression of FOXB2 gene of a stem cell, and determining the differentiated state based on the result", and "a differentiation marker selected from mRNA or protein derived from the FOXB2 gene". The present invention is applicable to quality management of the stem cells or to methods for preparation and isolation of the differentiated cells. Further, because the differentiated cells can be determined at an initial stage of culture, the present invention is useful for an early stage screening of cells and for quality management of the stem cells, and reduction in culturing period and cost reduction in expenditure regarding a culture medium, or the like, can be expected.