A single specimen of the slime flounder Microstomus achne (Pleuronectidae) was collected from the Iyo-nada Sea, western Seto Inland Sea, Japan in June 2009. The adult male specimen was 253.5mm total length, 211.3mm standard length. This species is known to inhabit the cold waters of northern Japan, but this is the first record of M. achne in the Seto Inland Sea, southwestern Japan.