The study sought to examine developmental processes of substitution in the pretend play of Down's syndrome children and to clarify the onset mechanism of subskills in decontextualization, which refers to the progress from imitation to substitution and then to invention. Eighteen Japanese children, aged 29-55 months, with Down's syndrome were given various lifelike objects, miniature toys, and ambiguous objects, and observed in their manipulation of the objects in play with their mothers. The developmental process of decontextualization in Down's syndrome children was the same as that in nonretarded children in previous studies. However, the onset of substitution showing double knowledge of objects (type 2) was delayed and the repertory of substitution hardly increased. Delayed emergence in type 2 substitution and little increment in the repertory of substitution suggest a relation to delayed language development in Down's syndrome children. (JDD)