This paper is concerned with the maximum power point tracking control of photovoltaic systems. In solar power generation using photovoltaic systems, DC-DC converters are usually inserted between photovoltaic systems and loads so that we can change operation points and thereby increase efficiency. However, the characteristic of the output power with respect to the duty ratio of DC-DC converters has multiple local maxima and drastically changes according to weather variation. In this paper, we propose an algorithm for online tuning of the duty ratio based on particle swarm optimization so that we can realize maximum power point tracking under such intractable circumstances. We finally verify the effectiveness of the algorithm by practical experiments.