To provide a box-shaped cab for a work machine, on which an operator operates the work machine, having a front surface portion, a rear surface portion, left and right side surface portions, a floor surface portion, and a ceiling surface portion, the cab for a work machine includes: a frame body having a pair of rear side structural members and a pair of lateral side structural members, a pair of rail members each having a vertical portion along the front surface portion and a horizontal portion along the ceiling surface portion; and a rectangular window frame being stored in the front surface portion or the ceiling surface portion, in which the vertical portions of the rail members are directly fixed to the lateral side structural members, and the horizontal portions of the rail members are fixed to the lateral side structural members via intermediate members.