A natural ventilation window (401, 601) comprising: a window frame (402, 602) which is installed in a building; a framed panel (404, 604) disposed in an opening portion (403, 603) defined by said window frame (402, 602); and opening/closing means (406, 606) for opening and closing the opening portion (403, 603) on the basis of an air current, said opening/closing means (406, 606) including a first supporting mechanism (432, 632) for supporting a portion (431, 631) on a top rail (422, 622) side of said framed panel (404, 604) liftably and rotatably about a horizontal axis and a second supporting mechanism (434, 634) for supporting another portion (433, 633) of said framed panel (404, 604) located below said portion (431, 631), movably in a direction toward an interior or exterior of the building and rotatably about the said another portion (433, 633) of said framed panel (404, 604) being disposed so as to be located above a center of gravity of said framed panel (404, 604) and to be located on a building exterior (407, 607) side with respect to that center of gravity when said framed panel (404, 604) is in an opening portion closed position, said second supporting mechanism (434, 634) including: a supported member formed in said another portion (433, 633) of said framed panel (404, 604); a support member (452, 652) for supporting said supported member linearly movably in the direction toward the interior or exterior of the building and rotatably about the horizontal axis; a rotating shaft (454, 654) four connecting said support member (452, 652) to said window frame (402, 602) rotatably about the horizontal axis so as to be located below and on a building interior (408, 608) side with respect to said supported member in a state of being supported by said support member (452, 652) linearly movably in the direction toward the interior or exterior of the building; a guide member (442, 642) and a holding mechanism (447, 647) for holding an angle of rotation of the support member (452, 652) in a state of supporting said supported member linearly movably in the direction toward the interior or exterior of the building, characterized in that; said guide member is fitted to said window frame (402, 602) and abuts against said supported member located close to the building interior (408, 608) side, so as to upwardly guide said supported member on the basis of the linear motion of said supported member in the direction toward the interior of the building and to downwardly guide said supported member on the basis of the linear motion of said supported member in the direction toward the exterior of the building, and said support member (452,652) includes: a linear motion supporting portion (453, 653) for supporting said supported member linearly movably in the direction toward the interior or exterior of the building and rotatably about the horizontal axis; and an end side supporting portion (457, 657) which is close to the building exterior (407, 607) side and is connected to an end portion (456, 656) close to the building exterior (407, 607) side of said linear motion supporting portion (453, 653), said end side supporting portion (457, 657) being adapted to support said supported member movably in a direction intersecting to the direction toward the interior or exterior of the building and rotatably about the horizontal axis, so as to displace said another portion (433, 633) of said framed panel (404, 604) downward at the time of movement of said another portion (433, 633) in the direction toward the interior of the building, and to displace said another portion (433, 633) of said framed panel (404, 604) upward at the time of movement of said another portion (433, 633) in the direction toward the exterior of the building.