A flexible base board for a chair, easily bends into a desirable shape. The base board fits more appropriately to the body of the person seated on the chair following the shape of the person's body without pressing the back, etc., thereby providing the person with good sitting comfort. The flexible base board is used for the backrest, etc. of a chair. The base board has an upper shell (4), an intermediate shell (5), and a lower shell (6). A large number of holes (14) having substantially the same size are substantially uniformly formed in the entire shell (6). Also, a large number of holes (15) are formed in the entire surface of the intermediate shell (5), and each hole (15) has a greater transverse width than a hole (14) in the upper shell (4) and the lower shell (6).