A cab reinforcement structure for reinforcing a cab by a gusset disposed in a corner at which a right rear pillar and a hollow rear lateral beam member joined to the right rear pillar meet each other is formed such that a gusset retaining member for sustaining a load from a gusset side is disposed within the rear lateral beam member in relation to the rear lateral beam member side end of the gusset and the gusset retaining member is composed of a plate-like member for blocking up the inside of the rear lateral beam member at a position corresponding to the rear lateral beam member side end of the gusset.