In the case of frictional contact problems subjected to periodic loading, due to non-uniqueness and non-linearity, the size of a load increment must be suitably determined to obtain the proper, physically meaningful path of response history. A method of an automatic load increment is proposed based on the sensitivity analysis of the solution with respect to the load scale parameter. Frictional contact conditions are formulated by complementarity and sensitivity analysis of the solutions with respect to the applied load, using the parametric optimal design theory. As illustrative examples, a layer pressed against a half-plane by a uniform pressure and subjected to a tangential force varying periodically in time is tested and compared with the solutions by Comninou and Barber (1983, Int. J. Solids Structures 19, 533 539). As a practical problem, a valve-cotter system of a small engine subjected to a periodic loading is solved as a three body contact problem.