MC5 molecule that exclusively expresses on the surface of embryonic mesenchyme cells (MCs) of starfish, Asterina pectinifera, newly comes within the category of metalloproteinase of astacin family. We studied the problem of how this novel membrane-type metalloproteinase functions during morphogenesis of the starfish. Inhibition experiments of MC5 molecule expression by use of Morpholino antisense oligonucleotides (MC5-MAO) reveals a phenotype as being "dwarfed effect"; those are, (1) Injection of the egg with MC5-MAO causes the larva to be significantly smaller, accompanied by a decrease in cell numbers and suppression of DNA-replication of epithelial cells (ECs) predominating at the body. (2) MC5-MAO injected larva displayed delayed development without inhibiting cell differentiation on nerve, muscle, and stomach cells once reaches the bipinnaria stage. These observations suggest that MC5 molecule positively works for ECs to proliferate. This hypothesis was proved by overexpression of MC5 mRNA and rescue-experiments co-injection with MC5 mRNA and MC5-MAO. Next, we examined the problem whether MCs involve in positive regulation for ECs proliferation. It is observed that although MC5-MAO induces "dwarfed effect" directly on the ECs, the phenotype becomes remarkable after ingression of the MCs into the blastocoel. Moreover, by injecting with MCs into the embryo showing "dwarfed effect", the embryo is shown to gain the DNA-replication meaningfully. Our results suggest that MCs are endowed with inducing activity for the ECs to proliferate during starfish morphogenesis.