This study is to focus on two constraints on WH-movement; one is the empty category prineiple (ECP) and the other is Subjacency Condition. And the experiment was conducted to examine (i) whether Japanese children show asymmetry between arguments and adjuncts on WH-movement, following ECP and (ii) whether Japanese children exhibit the distinction of islands with respect to the extradion of WH-phrases from islands. Another purpose of this study is to compare two different analyses of WH-questions in Japanese: Lasnik and Saito (1984, 1992) and Watanabe (1992), and this study also examines which analysis the experimental data support. The experimental data show (i) that Japanese children distinguished arguments from adjuncts on WH-movement and (ii) that they exhibited the distinction of two islands. The former one indicates that Japanese children follow ECP with respect to the interpretation of WH-questions. The latter one indicates that the data do not support the analysis by Lansnik and Saito (1984,1992).