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The electric quadrupole transition from the first 2 + state to the ground 0 + state in 18 C was studied through a lifetime measurement by an upgraded recoil shadow method applied to inelastically scattered radioactive 18 C nuclei. The measured mean lifetime is 18.9 ± 0.9(stat) ± 4.4(syst) ps, corresponding to a B(E2; 2 + 1 → 0 + g.s. ) value of 4.3 ± 0.2 ± 1.0 e 2 fm 4 , or about 1.5 Weisskopf units. The mean lifetime of the first 2 + state in 16 C was remeasured to be 18.3 ± 1.4 ± 4.8 ps, about four times shorter than the value reported previously. The discrepancy between the two results was explained by incorporating the γ -ray angular distribution measured in this work into the previous measurement. These transition strengths are hindered compared to the empirical transition strengths, indicating that the anomalous hindrance observed in 16 C persists in 18 C.