There is provided an auxiliary toilet seat with which a toddler can sit astride or sit in a posture like sitting on a chair. An auxiliary toilet seat main body of an auxiliary toilet seat is formed substantially oval. A seating surface of the auxiliary toilet seat main body is formed substantially flat. A splash prevention part is formed at each front and rear part of an inner peripheral part of the auxiliary toilet seat main body. An abutting fixing part having fixing position adjusting means is provided to each of two positions in a front part and rear part on a bottom surface of the auxiliary toilet seat main body. The auxiliary toilet seat is abuttable against a bottom surface of a toilet seat of a western style adult toilet bowl to be fixedly attached to the bottom surface.