The purpose of this study is to formulate the motion of a rigid body with unilateral contact problems by applying techniques of multibody dynamics and to analyze the issue of rocking condition of rigid bodies with slide contact. In To investigate rocking motion with slide contact, we formulate for dynamics of a simple rigid body system with a unilateral contact model. Judgment for the occurrence of contact between a rigid body and a base is applied. The planar motion of a rigid body system having a simple shape and both with and without slide cases is assumed. Using constraint conditions for the contact as algebraic equations, the rocking motion of the rigid body, including slide and frictional force, is analyzed. The differential algebraic equation is solved by the augmented method with Lagrange multipliers, using generalized coordinates and independent variables that describe the contact points. The influence of the frequency and amplitude of disturbance given to the base is discussed.