A device for the opening/closing of a window casing, the casing comprising a fixed frame and a hung sash connected to said fixed frame pivotally around an axis of rotation, the hung sash having a free upright parallel to and spaced apart from said axis of rotation, the device comprising: engagement means mounted on said fixed frame; a rod adapted to slide in a reciprocating manner along according to a sliding direction (X') along said free upright, between an insertion position and an unlock position, said positions corresponding to the locking and the unlocking of the sash with respect to the frame in a closing arrangement of the casing, through respectively the engagement and disengagement of latch means associated with the rod with said engagement means; a handle to drive the sliding of said rod between said insertion and unlock positions; an arm that limits the rotation of said hung sash with respect to said fixed frame during the opening movement, opposite ends of said arm being respectively connected with a pivotal coupling to said fixed frame and with a roto-translatory coupling to said sash, the roto-translatory movement including a translation in the sliding direction (X') of the rod from two end positions one of which corresponds to the stop of the hung sash in the configuration of maximum opening with respect to the fixed frame; said device comprising locking means (8, 80) for locking the position of said arm with respect to said hung sash in the end position corresponding to the maximum opening configuration, said locking means being directly engaged with and driven by said rod and being adapted to engage with said arm to obtain its lock in response of a sliding movement of said rod towards said insertion position, wherein the roto-translatory coupling between said arm and said free upright comprises a pin integral with the arm and a slot formed in said free upright, said slot comprising a main straight segment extending along said sliding direction (X') and at least one end segment diverging from said main segment, said pin being adapted to become arranged at a run stop in said end segment when the hung sash is in its maximum opening configuration, said locking means further comprising an end member fixed with the rod, said end member comprising at least one abutment element adapted to become aligned transversally, i.e. according to a direction orthogonal to said sliding direction (X'), with said pin to mechanically interfere with the same pin to prevent it from coming out of said divergent end segment thus avoiding a resulting accidental closing of the hung sash when the rod is in said insertion position, said slot being formed in a flat body connected to said free upright and defining in cooperation with the upright itself a seat adapted to slidingly house, in a mutually superimposed fashion, said end member and an enlarged head of said pin, wherein said enlarged head of said pin provides a main side exceeding in length the width of said slot, and a smaller side comparable or smaller than the width of the slot, so that when the main side of the head is aligned with the main straight segment of said slot, the pin can be disengaged from said slot and consequently from said upright.