A novel differential in-pipe robot, called mechanical adaptive in-pipe robot is presented. The drive unit of the robot consists of driving module characterized by tri-axis differential velocity mechanism, and elastic leg-wheeled in-pipe adaptability module characterized by fully active drive. The drive module, as a result of its automatically modulating every moving wheel rotation speed according to topological constraint by in-pipe, avoids parasite power as a result of moving wheel slippage. The elastic leg-wheeled in-pipe adaptability module is adjustable to inner diameter variation by its radial protraction and shrinkage, and its fully active structure ensures enough tractive force even if several moving wheels impended. Therefore the mechanical adaptive in-pipe robot provides a novel insight to differential-typed in-pipe robots with the mechanical adaptive performance, and is supplementary with its novel configuration and transmission principle.