We have successfully opened the way to understanding the mechanism of peculiar reactions taking place in supercritical water. One of reactions in question refers to the Beckmann rearrangement where ε-caprolactam is produced from cyclohexanone oxime in supercritical water without using concentrated sulfuric acid, which was experimentally demonstrated by the SFRC-AIST. It has been successfully demonstrated, first in the world by the computer simulation based on the first principle molecular dynamics, that the rearrangement is made through incomplete hydrogen bond network created owing to peculiar density property of supercritical water, neither liquid nor gas, but an intermediate state between them.