An auxiliary toilet stool seat adapted to be mounted on a seat of a western-style toilet stool, the auxiliary toilet stool seat having a main body portion which is formed from a hard resin into an annular shape having an angled cross section made open at a bottom and having a flat top portion, a first leg portion which is brought into abutment with a top surface of the auxiliary toilet stool seat, a second leg portion which is slidably movable in such a manner as to follow an inner perimeter of the seat, and a third leg portion which is formed into an L-shape and is brought into abutment with a rear surface of the seat on a horizontal surface of the L-shape, the first, second and third legs being provided on a rear surface of the main body portion of the main body portion.