The purposes of this study were twofold: to investigate the extent of the disturbance of length conservation by optical illusions, and the ways of solving or dissolving contradictions between real and apparent length. Subjects were 55 kindergarteners and 83 school children. The main results follow: 1) The optical illusions were effective in nonconservers ; with advancing in age, the effects of these illusions decreased ; 2) Five types of coping with the contradictions were found out: I-Impossible, II-Unappropriate dissolving, III-Appropriate dissolving, IV-Particular solving, and V-General solving. Type I largely appeared in 4-8 years old while suddenly decreased in 9-10 years old. Type IV showed sudden increase in 9-10 years old and type V in 11-12 years old. These findings showed that around 9-10 years of age, the distinction between real and apparent length became possible, and two years later the distinction was generalized.