A cylinder body 60 of a wheel cylinder assembly 6 for expanding a pair of brake shoes 3 and 4 by a pair of pistons 31 and 32 brought into contact with the respective brake shoes has a shape of a single straight pipe with both ends which are opened. The cylinder body 60 includes a first cylinder portion 35 for the one 32 of the pistons and a second cylinder portion 34 for the other 31 of the pistons. A valve seat 43 of a control valve 39 is arranged between the cylinder portions 34 and 35.