To provide a structure for fixing a pillar-to-pillar member, which can reduce vibrations propagated to steering.

A fixing structure 10 is formed by connecting a floor brace member 30 which is installed in such a manner as to rise upward from a floor surface side of a vehicle, to a pillar-to-pillar member 20 in a vehicle-width-direction intermediate part. In addition, the floor brace member 30 includes a rising part 32 which is formed in a shape elongated in a vertical direction, and a partitioning surface 36 which vertically partitions a space 38 formed inside the rising part 32. The floor brace member 30 can attenuate the vibrations which are propagated from the pillar-to-pillar member 20, by the resonance of the rising part 32 and the partitioning surface 36.