This review concerns Professor Hitoshi Nagai's recent book, Wittgenstein no Goshin (Wittgenstein's Misdiagnosis). In this book, Professor Nagai makes his understanding of the latter part of Wittgenstein's “Blue Book”. He argues that Wittgenstein set up an epoch-making solipsism, which Professor Nagai calls “Wittgenstein's solipsism” or “linguistic solipsism”, but he adds that Wittgenstein himself misunderstood the meaning and significance of this solipsism. In this review, I consider the validity of Professor Nagai's arguments.