According to the present invention, when cycloolefins are produced by partially hydrogenating a monocyclic aromatic hydrocarbon with hydrogen in the presence of a ruthenium catalyst, water, and a metal sulfate, the decrease in catalytic activity and cycloolefin selectivity is suppressed by reducing the concentration of chloride ions dissolved in the water in which the catalyst is present to 300 wt ppm or less and regenerating a part or all of the catalyst for reuse. Zinc sulfate is preferably used as the metal sulfate. Further, the raw materials and catalyst to be supplied to the reaction preferably have a reduced chloride ion content. This method can suppress the decrease in long-term catalytic activity and cycloolefin selectivity. Furthermore, the catalytic activity and cycloolefin selectivity after catalyst regeneration can be maintained at a high level. As a result, cycloolefins can be efficiently produced for a long period of time.