Preliminary results of specific heat measurement in applied magnetic fields on the filled skutterudite CeOs₄Sb₁₂ are presented. Clear anomaly in the specific heat data suggests the existence of an intrinsic phase transition at 0.9 K, accompanied by a gap opening in moderately mass-enhanced quasiparticle bands. Unusual magnetic field effect on the transition temperature, which shifts toward higher temperatures with increasing field, is revealed.

PACS numbers: 75.40.Cx, 71.27.+a, 75.30.Kz, 75.30.Mb