Fruit size depends on cell division and cell expansion, and fruit development comprises cell division and cell expansion periods. Here we focused on the regulatory mechanism of the cell division as a determining factor for fruit size in tomato as the model plant of fruit crops. In Arabidopsis roots, auxin-induced HIGH PLOIDY 2 (HPY2) reportedly regulates cell division. In this study, we performed auxin and anti-auxin treatments, and the expression of the tomato HPY2 homolog SlHPY2 was analyzed to reveal the effect of auxin on cell division in tomato fruit. SlHPY2 mRNA levels were higher at the cell division stage. The number of cells increased and decreased by auxin and anti-auxin, respectively, and the SlHPY2 mRNA levels increased by auxin. Auxin also increased the mRNA levels of tomato cyclin-dependent kinase. The results suggest that cell division during early fruit development is promoted by SlHPY2 gene expression enhanced by auxin. Interestingly, the expression of the tomato CCS52A homolog SlCCS52A as an index of endoreduplication increased by auxin, suggesting that the role of auxin in endoreduplication could be different between tomato fruit and Arabidopsis roots.