To provide a coil bobbin for a transformer in which no through hole is formed in both sides of an upper flange to thereby prevent upsizing, and no long arm apt to splinter is formed on a terminal board, and the terminal board can be readily mounted to a flange. In the coil bobbin in which an end of an upper flange (3) formed on an upper end of a drum (2) is formed with a terminal board mounting portion (5), and in which a separately formed terminal board (8) is mountable on the terminal board mounting portion (5), an inside of the terminal board mounting portion (5) is formed with a rib (7), whose upper portion is formed with a plurality of plate-like walls, and an inner end of the terminal board (8) is formed with a plurality of through holes (17) in which said walls are inserted.