A novel organic anion transporter gene participating in organic anion transport in the placenta; and an organic anion transporter which is a polypeptide encoded by the gene. A placental organic anion transporter OAT4, more particularly, a placental organic anion transporter OAT4 having the amino acid sequence represented by SEQ ID NO:2 or an amino acid sequence derived therefrom by deletion, substitution or addition of a part of the amino acids thereof. A nucleic acid (preferably DNA) having a base sequence encoding the placental organic anion transporter OAT4 or a base sequence hybridizable therewith under stringent conditions.