An object of the present invention is to provide a simple and efficient device for predicting a risk of occurrence of a side effect of irinotecan by analyzing a single nucleotide polymorphism in a region encoding a specific gene. The prediction of the risk of the occurrence of a side effect of irinotecan is assisted by analyzing a single nucleotide polymorphism in a region encoding the APCDD1L gene, the R3HCC1 gene, the OR51I2 gene, the MKKS gene, the EDEM3 gene, or the ACOX1 gene which are present on genomic DNA in a biological sample collected from a test subject; or a single nucleotide polymorphism which is in linkage disequilibrium with or genetically linked to the single nucleotide polymorphism, and determining whether the single nucleotide polymorphism is homozygous for a variant type, heterozygous, or homozygous for a wild-type.