The awareness of one's comprehension failure has been researched on problem solving in mental retarded children. Past studies suggested that non-awareness of one's comprehension failure in a given ambiguous instruction, without investigating how much retarded children would understand ambiguous instructions. In the present study, the comprehension of ambiguous instruction was investigated on children with and without mental retardation matching them on MA. Each child was instructed to solve a problem containing an ambiguous instruction for problem solving. Retarded children at MAs of 7 years were unaware of the ambiguity ; they understo od the instruction in their own context. In contrast, the retarded children at MAs of 11 yea rs and non-retarded children at both MAs were aware of it, had ideas about the intent ion of the one setting the problem, and rethought about the task request. In addition, another instruction was presented to allow a second thought on the task request. Almost all retarded children showed more performance by means of such instru ction.