Inverted DI cans are fed by a conveyer having partitions in a plurality of rows such that they are spaced apart in each row, and treatment liquid is sprayed against the travelling cans from above and below the center of each row. The liquid is sprayed from above in a uniform and a full-cone pattern greater in area than the top surface of the can and from below also in a full-cone pattern or in a fan-shaped pattern narrow in the widthwise direction of the conveyer and greater in length than the can open end diameter. The liquid is further sprayed against the travelling cans from side nozzles on the opposite sides of and symmetric with respect to the center of each row. The side walls of the cans are thus washed without contact of adjacent cans in the direction of travel of the cans. The washing force is increased in the space between adjacent cans in the direction of travel to prevent washing irregularities and thus permit uniform surface treatment of the inner and outer surfaces of the cans.