Use of mordenite zeolite and zeolite Y as adsorbents enables a dimethylnaphthalene isomer mixture to be efficiently separated. A feedstock oil containing a dimethylnaphthalene isomer mixture including an .alpha.,.alpha.-isomer, an .alpha.,.beta.-isomer, and a .beta.,.beta.-isomer is passed through an adsorbent layer (A) including mordenite zeolite together with a developing solvent. Subsequently, a liquid discharged from the adsorbent layer (A) is passed through an adsorbent layer (B) including zeolite Y. An eluent is passed through the adsorbent layer (B). The solvent is removed from the resultant eluate to obtain the .alpha.,.beta.-isomer of dimethylnaphthalene. Similarly, the .beta.,.beta.-isomer of dimethylnaphthalene is obtained from the adsorbent layer (A).