SemiQuad is walking robot prototype with a platform and two two-link legs. Thus, it is five-link mechanism. Front leg models identical motions of two quadruped's front legs, rear leg models identical motions of two quadruped's rear legs. We call this robot also as “virtual” quadruped. The mechanism is driven by four DC motors. Its control system comprises a computer, hardware servo-systems and power amplifiers. The locomotion of the prototype is planar curvet gait. In the double support our prototype is statically stable mechanism. In the single support it is unstable and under actuated system. There is no fly phase. We describe here the scheme of the mechanism, the characteristics of the drives and the control strategy. The dynamic model of the planar walking is recalled for the double and single support phases. The designed control strategy overcomes the difficulties appeared due to unstable and under actuated motion in the single support. In the first experiments, the designed algorithm was successfully implemented.