The aim of this study was to develop a quadruped robot focusing on the steady control structure of the trunk. The approach is to combine merits of analog and digital logic circuits. The analog circuits were used for the gait control and digital circuits were used for the balancing control of the trunk. A quadruped robot was developed, named NTU-SIMONE. The balance of the trunk can be maintained when one leg raised up to 17cm in vertical direction.