Disclosed is a method for predicting whether the esophagus cancer is sensitive or resistant to a therapy using an ERBB3 inhibitor (i.e., an anti-ERBB3 antibody). Specifically, the method carries out prediction by measuring expression of the RNA level or protein level of a marker of at least one of SDC, PTGES, NCF2, NOXA1, CARD6, and GNAZ in a tumor sample.