A lock system is made in a combination of a rotatable latch and a release lever for restricting the turn of the latch in a disengaging direction. The lock system further includes a release lever disengagement maintaining mechanism which engages with the release lever to maintain it in its disengaging condition wherein the latch is disengaged from the release lever. The release lever disengagement maintaining mechanism is designed to release the release lever from its disengaging condition when the latch turns through a specified rotation angle. Lock mechanisms are provided for a lift open type window which can be housed by moving a movable window frame upward from a window closing upright position, being guided by a pair of guide rails. The lock mechanisms are respectively attached to both ends of a top part of the window frame, so as to be exterior to the window frame and interior to and under the guide rails. Each lock mechanism has a turnable latch having two engagement notches at positions which are a specified rotation angle displaced from each other and a release lever for restricting the turn of the latch and for disengaging the latch. There are also provided strikers at the upper part of a window frame's home position where the window frame stands up and at the end of a window housing position. These strikers are respectively attached to a support structure so as to be parallel with the axis of rotation of the latch, and respectively come into engagement with their corresponding engagement notches of the latch to hold the window frame.