To exchange filters in a state where water tanks disposed on uneven steps or the like are used by arranging the filters including a purifying agent on wall surfaces at the upstream side and the downstream side of flower baskets disposed inside the tanks so as to be inserting/pulling-out possible from the upper part of each basket.

Each flower basket 13 formed by a punching net is arranged inside each water tank 12, which has rails for inserting the filters 15 including the purifying agent such as zeolite or activated carbon at the both ends. A frame body is inserted to the basket 13, which consists of a transparent plastic material to prevent cut flowers being a merchandise from being separated. Then, the flow-in port 16 and the flow-out port 17 of cold water are arranged at the left side of the tank 12, the flower basket 14 is disposed inside the tank 12 and the filters 15 are arranged on the left and right walls 18 of the basket 13 so as to be inserted or pulled out of the upper part. The filters 15 are formed in a cassette shape. Then, they are exchanged with the preliminary filter and with the clean one as necessary.