In the paper, overall procedures of developing a mult-legged underwater robot inspired by diving beetles are described. The research has been conducted by dividing it into two fields in terms of biomimetics(Swimming locomotion and structural analysis of the diving beetle’s leg). Firstly, we propose the established SPG(Swimming Pattern Generator) mimicking locomotion of diving beetles with a view point of bimomimetics for legged underwater robots. Firstly, the locomotion of the diving beetle has been observed and classified through experiments with a motion capture system consist of high-speed camera and imaging processing software Image J. Subsequently, we analyzed coordinated patterns of rhythmic movements of the diving beetle and formulate equations corresponding to the raw data of each joint, which we obtained through the motion capture system, by employing Fourier mean square fitting method. Based on this, control parameters have been determined by understating their characteristics through the procedure of comparing produced swimming trajectories according to varying their values to observed motions of the diving beetle. As an extended research for SPG, the parameters are expressed in a parameter space based on correlations of individual parameters and a newly configured parameter space is proposed. Consequently the number of parameters has been successfully reduced for improving controllability of SPG. Additionally, it is described that the design of a bio-inspired legged underwater and estimating its performance by implementing simulations. To make it possible for the robot to both walk and swim, the transformable kinematic model according to applications of the leg is proposed. In order to aid the robot development and estimate swimming performance of the robot in advance, an underwater simulator has been constructed and an approximated model based on the real robot has been set up in the simulation. Through the results, we established the strategy of leg joints which make the robot swim in the three dimensional space to reach effective controls. Furthermore, designing process for the robot based on the structural analysis of the diving beetle and system architecture of the robot are explained. Structural advantages(passive joints and bristles on the leg) to maximize propulsion and minimize water resistance for efficient swimming of the diving beetle have been applied to designing robot legs. Performances of the leg are verified by measuring drag force by a force/torque sensor through underwater experiments. Lastly, underwater experiments, which bio-mimicking locomotion was applied to the robot, have been conducted to confirm the swimming performance.