A bicycle seat post fixing structure includes a tubular attachment member and a tightening member. The tubular attachment member is configured to be fixed to the frame. The tubular attachment member includes a first threaded component and a first inner peripheral surface configured to receive the seat post. The tubular tightening member includes a second threaded component that mates with the first threaded component and a second inner peripheral surface configured to receive the seat post. One of the first and second threaded components is a female threaded component and the other of the first and second threaded components is a male threaded component having at least one slit formed therein. At least one of the male and female threaded components has tapered threads such that rotation of the tightening member in a tightening direction squeezes the male threaded component to hold the seat post.