We investigated the growth inhibitory effect of internal organs of Aplysia kurodai (AK) on proliferation in cancer cell lines in vitro. The internal organs of AK were extracted with methanol (AKM), which were then further fractionated into four subfractions by using solvent partition method, resulting in hexane (AKMH), methanol (AKMM), butanol (AKMB), and aqueous (AKMA) soluble fractions. We determined the cytotoxic effect of these four fractions in four kinds of cancer cell lines - HepG2, MCF-7, HT29 and B16-F10 - by MTT assay. Among the four subfractions of AKM, AKMM showed the strongest cytotoxic effects on all cancer cell lines which were used. Morphological changes such as membrane shrinking and blebbing of cells were also observed in AKMM treatment in HepG2 cells. In addition, we also observed quinone reductase (QR) induced effect in the methanol layer (AKMM) of HepG2 cells. AKMM showed the highest induction activity of quinone reductase on HepG2 cells among the partition layers. The QR induced effect of AKMM was determined to be 2.4 at $100;{mu}g/ml$ level with a control value of 1.0. Although further studies are needed, the present work suggests that internal organs of Aplysia kurodai (AK) may be a chemopreventive agent for the treatment of human cells.