To produce a three-dimensional fiber structure curved along the longitudinal direction and usable as the optimum reinforcing material for a fiber-reinforced composite material for a curved beam, etc.

A linearly extending three-dimensional fiber structure F having a profile cross-section with multiple plates forming right angles is clamped with an assisting member 19 having a face corresponding to the multiple plates. Form halves 17a, 17b constituting a press form are moved with an actuator to bend the three-dimensional fiber structure F at a prescribed curvature while keeping the cross-sectional form of the structure F and the structure F is pressed to obtain the three-dimensional fiber structure 11 curved in the longitudinal direction.