There have been intensive researches on insect flight during recent decades, studying aerodynamic principles of flapping flight. The objectives of this study are to understand the flying mechanism and hind wing morphology of a beetle insect in free forward flight. The three-dimensional wing kinematic as well as marvelous wing deformation is presented in an experimental set. The flight mechanism of beetle in free forward flight is explored by three-dimensional numerical simulation using structured in-house code, KFLOW. Roles of camber and local corrugation on hind wing are clarified for aerodynamic forces generation.