PROBLEM TO BE SOLVED: To find an active ingredient having an excellent neutral fat accumulation-inhibiting action and to provide a neutral fat accumulation inhibitor, and a slimming agent.SOLUTION: The neutral fat accumulation inhibitor or the slimming agent having an excellent effect contains, as an active ingredient, an extract of one or more shellfishes selected from Tapes philippinarum, Fulvia mutica, Patinopecten yessoensis, and Atrina pectinata japonica with a water-soluble solvent.