Use of a mixing device for mixing stock oil with a particulate catalyst, comprising: a moving bed forming unit that forms a cylindrical moving bed by continuously dropping the catalyst from the top to the bottom while distributing the catalyst in an annular form; a reaction tube that surrounds the moving bed; an internal stock oil supply part that has an internal stock oil injection nozzle (34, 234) for supplying the stock oil over the entire area in a circumferential direction of an inner surface of the moving bed; an external stock oil supply part (40, 240, 340) having an external stock oil injection nozzle (48, 248) for supplying the stock oil over the entire area in a circumferential direction of an outer surface of the moving bed, and means for setting a mass flow and velocities characterized in that when the mass flow per unit area of the moving bed is Q [kg/m 2 s], the difference between an outer diameter and an inner diameter of the moving bed is W [m], an inner diameter of the reaction tube is D [m], a linear velocity of a horizontal component of the stock oil at a jet orifice of the internal stock oil injection nozzle (34, 234) is u1 [m/s], the stock oil being supplied from the internal stock oil supply part, and the linear velocity of a horizontal component of the stock oil at a jet orifice of the external stock oil injection nozzle (48, 248) is u2 [m/s], the stock oil being supplied from the external stock oil supply part (40, 240, 340), the following identities 8.0 ≥ Q x (W/D) / (u1 + u2), Q = 300 to 2000 [kg/m 2 s], W/D = 0.2 to 0.5, u1 = 5 to 300 [m/s], and u2 = 5 to 300 [m/s] are satisfied. a moving bed forming unit that forms a cylindrical moving bed by continuously dropping the catalyst from the top to the bottom while distributing the catalyst in an annular form; a reaction tube that surrounds the moving bed; an internal stock oil supply part that has an internal stock oil injection nozzle (34, 234) for supplying the stock oil over the entire area in a circumferential direction of an inner surface of the moving bed; an external stock oil supply part (40, 240, 340) having an external stock oil injection nozzle (48, 248) for supplying the stock oil over the entire area in a circumferential direction of an outer surface of the moving bed, and means for setting a mass flow and velocities characterized in that when the mass flow per unit area of the moving bed is Q [kg/m 2 s], the difference between an outer diameter and an inner diameter of the moving bed is W [m], an inner diameter of the reaction tube is D [m], a linear velocity of a horizontal component of the stock oil at a jet orifice of the internal stock oil injection nozzle (34, 234) is u1 [m/s], the stock oil being supplied from the internal stock oil supply part, and the linear velocity of a horizontal component of the stock oil at a jet orifice of the external stock oil injection nozzle (48, 248) is u2 [m/s], the stock oil being supplied from the external stock oil supply part (40, 240, 340), the following identities 8.0 ≥ Q x (W/D) / (u1 + u2), Q = 300 to 2000 [kg/m 2 s], W/D = 0.2 to 0.5, u1 = 5 to 300 [m/s], and u2 = 5 to 300 [m/s] are satisfied. 8.0 ≥ Q x (W/D) / (u1 + u2), Q = 300 to 2000 [kg/m 2 s], W/D = 0.2 to 0.5, u1 = 5 to 300 [m/s], and u2 = 5 to 300 [m/s] are satisfied.