It is intended to provide a method for determining a tumor. The method for determining a tumor comprises: (1) treating genomic DNA prepared from a subject tissue or cell with bisulfite (the subject tissue or cell is derived from a patient who is affected by a tumor and is determined as (i) having MSI-H of the tumor in MSI examination and/or no or reduced expression of MLH1 in the tumor in immunohistochemical examination, and (ii) having no mutation in MLH1 in genetic examination); 2) amplifying, by PCR, DNA comprising a portion or the whole of MLH1 promoter region from the bisulfite-treated DNA; 3) subjecting the PCR amplification product to ion exchange chromatography to obtain a detection signal; 4) determining whether or not the peak of the detection signal is a peak indicating highly methylated DNA; and 5) determining the tumor as a tumor derived from a patient without Lynch syndrome when the peak is determined as a peak indicating highly methylated DNA.