To provide a method for producing an oxidized fiber structure good at shapability and suitable as a raw material for high-density and thin carbon fiber structure, and to provide a method for producing such a carbon fiber structure.

This method for producing the oxidized fiber structure 12 comprises oxidizing a precursor structure 4 under continuous thermocompressive molding in 200-350°C air at a planar pressure of 0.01-1 MPa under a tension of 5-500 kPa.cm using a thermocompressive oxidation processing apparatus 6. The other objective method for producing the carbon fiber structure comprises carbonizing the oxidized fiber structure 12 at ≥500°C in an inert gas using a carbonizing apparatus 16.