To keep a catalyst moderately damp and to prevent the generation of dust when the catalyst used in a catalytic reaction of hydrocarbons is drawn from a reaction tower by introducing a washing oil having a specified distillation temp. and a specified kinematic viscosity into the reaction tower to wet the catalyst.

A washing oil having 300-460°C 50% distillation temp. and 1.5-5.0 mm2/s kinematic viscosity at the filling stop temp. of the washing oil is successively circulated in a reaction tower to wash and wet a catalyst used in a catalytic reaction of hydrocarbons. That is, the temp. of the reaction tower is first dropped to 250-350°C, the filling oil is changed over from the stock oil to the washing oil and the catalyst is washed and wetted. Gaseous hydrogen or the like is then introduced and the oil in the catalyst is removed in the atmosphere of the gas. At the time when the temp. of the reaction tower is dropped to ordinary temp. to ≤70°C, the tower is degassed to atmospheric pressure, combustibles such as gaseous hydrogen is removed, the tower is opened and the catalyst is drawn.