To clarify the effects of surfactant solutions on the drag coefficient of a circular cylinder, the flow past a circular cylinder was investigated at 10<Re<7×103 by measuring the drag and by visualizing the flow. Six cylinders with diameters between 2 and 20 mm were tested. Surfactant solutions were aqueous solutions of oleyl-methyldihydroxethyl ammonium choride in the concentration range of 50 to 200 ppm with sodium salicylate. It was clarified that the drag coefficient of the cylinder in surfactant solutions increases comparing that in tap water in the lower Reynolds number range. On the contrary, the drag coefficient decreases in higher Reynolds number range. The maximum drag reduction ratio was approximately 55% for 200 ppm solution at Re=7×103. The flow visualization results showed that the drag of surfactant solutions increases because of the existence of the wide stagnant zone around the cylinder. In addition, the numerical simulation results by using the FENEP model showed that the increase of apparent shear viscosity is an important factor of the generation of the stagnation zone.