A slat supporting member (8), which is interposed between, on the one hand, a pair of link mechanisms (4,5) and, on the other hand, each of a plurality of slats (9) to allow said slat (9) to be supported by said pair of link mechanisms (4,5), has a first support and a second support connected to the first support by means of a fitting and securing device. The first support includes a first arm portion having a pair of shaft holes in which shaft portions of a pair of shaft members (23,26) are respectively inserted and secured rotatably, a shaft portion disposed in such a manner as to pass through a guide slit, and a fitting projection adapted to be fitted in a fitting portion of said slat. The second support includes a second arm portion having at one end portion and the other end portion thereof a pair of shaft holes in which shaft portions at the other end portions of the shaft members (26) are respectively inserted and secured rotatably.