[en] We found a metamagnetic like anomaly at H_m ≅ 5 kOe in a heavy fermion compound YbCo₂Zn₂₀ below the characteristic temperature T_χmax=0.32 K where the ac-susceptibility shows a broad peak, suggesting that an electronic state with a very low Kondo temperature is realized. Interestingly, the metamagnetic like behavior was observed as two peaks at 4.0 and 7.5 kOe at 95 mK in the magnetic field dependence of the electronic specific heat C/T. The extremely large values of the electronic specific heat coefficient γ≅8000 mJ/(K²·mol) and A=160 μΩ·cm/K² in the electrical resistivity ρ=ρ₀+AT² at H=0 kOe are most likely due to the very low Kondo temperature. The √A value was, however, found to be strongly reduced from √A=12.6 (μΩ·cm/K²)^1/2 at 0 kOe to 0.145 (μΩ·cm/K²)^1/2 at 150 kOe. Therefore, we considered that the corresponding cyclotron effective mass m_c^*, which was determined from the temperature dependence of the de Haas-van Alphen (dHvA) amplitude, is also reduced with increasing magnetic field and is in fact not large, ranging from 2 to 9m₀ at 117 kOe. From the field dependence of √A and m_c^*, we estimated the cyclotron effective mass at 0 kOe to be 100-500m₀, revealing the largest cyclotron mass as far as we know. (author)