A roof skylight window comprising: - a window casement frame that, in use, demarcates a window casement opening; - a tiltable window frame; whereby the tiltable window frame is incorporated in the window casement frame, tiltable around a pivot center-line between a closed position, wherein the window casement opening is closed, and an open position wherein the window casement opening is at least partially open; whereby the tiltable skylight window frame comprises a rectangular window frame composed of four frame members with a glass pane incorporated therein; whereby the tiltable skylight window further comprises: - a ventilation passage provided in one of the frame members; - a cover plate which is moveable between a closed position closing the ventilation passage and an open position releasing the ventilation passage; - a first locking member moveable between a locking position and a disengaging position, whereby the first locking member in the locking position of the tiltable window frame engages the window casement frame in order to lock the tiltable window frame in relation to the window casement frame, and whereby the first locking member permits the tilting of the tiltable window frame around the pivot center-line in the disengaging position; - an operating member having a longitudinal axis, which operating member is operationally connected to both the cover plate and the first locking member so that, in operation, the first locking member and the cover plate can be moved between a first operating position in which the cover plate and the first locking member are respectively in the closed and locking position, and a second operating position in which the cover plate and the first locking member are respectively in the open and disengaging position; wherein, the roof skylight window is further provided with a second locking member having a longitudinal axis, which second locking member is moveable along its longitudinal axis between a locked position and a disengaged position; whereby, on the one hand, the second locking member engages the operating member in the locked position and the window casement frame or the rectangular window frame on the other hand for locking the operating member in the first operating position, and whereby the second locking member in the disengaged position releases the operating member for operation, and wherein the longitudinal axis of the second locking member and the longitudinal axis of the operating member are substantially parallel for extensive integration of the second locking member and the operating member, and wherein the second locking member is slideably connected with the operating member. - a window casement frame that, in use, demarcates a window casement opening; - a tiltable window frame; - a ventilation passage provided in one of the frame members; - a cover plate which is moveable between a closed position closing the ventilation passage and an open position releasing the ventilation passage; - a first locking member moveable between a locking position and a disengaging position, whereby the first locking member in the locking position of the tiltable window frame engages the window casement frame in order to lock the tiltable window frame in relation to the window casement frame, and whereby the first locking member permits the tilting of the tiltable window frame around the pivot center-line in the disengaging position; - an operating member having a longitudinal axis, which operating member is operationally connected to both the cover plate and the first locking member so that, in operation, the first locking member and the cover plate can be moved between a first operating position in which the cover plate and the first locking member are respectively in the closed and locking position, and a second operating position in which the cover plate and the first locking member are respectively in the open and disengaging position; wherein, the roof skylight window is further provided with a second locking member having a longitudinal axis, which second locking member is moveable along its longitudinal axis between a locked position and a disengaged position;