A sash window includes a support device disposed on a lower end portion of each sash and including a pivot shaft for pivotally supporting the sash relative to a window frame, and a latch device disposed on an upper end portion of the sash for interlocking engagement with each jamb member to lock the sash in position against pivotal movement relative to the window frame. The window frame has a holder member for holding the pivot shaft in position against movement in inward and outward directions of the sash window when the sash is in a fully closed position. A first reinforcement member is mounted in at least one sash rail, and a second reinforcement member is mounted in a portion of each jamb member adapted for engagement with the latch device in the fully closed state of the sash, so as to indirectly reinforce the jamb member portion from inside the jamb member.