The disclosure describes a sash window assembly that allows a front-end lock to be locked easily simply by closing a sash. The sash window assembly includes a guide member that is provided on one of either an opening frame and a front-end frame element, and that comes into contact with the other of either the opening frame and the front-end frame element as the sash is being closed and guides thefront-end frame element to a predetermined position, in a depth direction of the sash window assembly,with respect to the opening frame. A lock receptacle member includes an abutted portion that is abutted by a lock body as the sash is being closed, and an engagement portion with which the lock body engages. The lock body includes a hook member having an abutting portion that abuts against the abutted portion due to the sash being closed, and a hook portion that engages with the engagement portion by turning about a turn axis due to the abutted portion abutting against the abutting portion. When or before the abutting portion abuts against the abutted portion, the other of either the opening frame and the front- end frame element comes into contact with the guide member provided on the one of either the opening frame and the front-end frame element.