An antiferroquadrupolar ordering at T_Q=0.11K has been found in a Pr-based superconductor PrIr_2Zn_<20>. The measurements of specific heat and magnetization revealed the non-Kramers Γ_3 doublet ground state with the quadrupolar degrees of freedom. The specific heat exhibits a sharp peak at T_Q=0.11K. The increment of T_Q in magnetic fields and the anisotropic B-T phase diagram are consistent with the antiferroquadrupolar ordered state below T_Q. The entropy release at T_Q is only 20% of Rln2, suggesting that the quadrupolar fluctuations play a role in the formation of the superconducting pairs below T_c=0.05K.