To provide a drop prevention mechanism for a lift sash window included in a sealing device, capable of more reliably and stably performing drop prevention.

A vertical sash frame of a lift sash window in which a drop prevention mechanism 31 is installed includes a groove in which a surface facing a vertical frame having corresponding positional relation is opened. The fall prevention mechanism 31 includes at least: a connection metal fitting 32 connected to a hanging and support unit 18 side and disposed in the groove; a latch body 42 including a latch part 44 at a top end side directly facing an open side of the groove and connected to the connection metal fitting 32 by regulating rotation of a base end side by a rotation regulating member; and a weight body 55 connected to the latch body 42 in such balance that the entire latch body 42 is accommodated in the groove in a state of receiving tensile force from the hanging and supporting means 18 and the latch part 44 of the latch body 42 automatically protrudes from the inside of the groove in a state of not receiving the tensile force. The vertical frame is provided with step parts latched with a lower surface of the latch part 44 of the latch body 42 during falling at proper intervals.