A museum showcase comprising a casing, at least one sliding door and a guide system for a sliding door, wherein the guide system comprises an upper guide mechanism and a lower guide mechanism, and wherein the door is hung on the casing through the upper guide mechanism, which bears the weight of the door while the lower guide mechanism determines the inclination of the door in relation to a vertical direction, wherein the upper guide mechanism comprises: a primary rail and a secondary rail, placed side by side and extended parallel in a same opening direction for opening the door, wherein the primary rail is fastened to the casing and is extended between a first end and a second end, and the secondary rail is fastened to the door at one of the upper edges thereof and is extended between a first end and a second end, wherein the secondary rail is mounted sliding in relation to the primary rail along said opening direction, in such a way that it can slide between a closed position of the door and an open position of the door, wherein in the closed position of the door the first and the second end of the secondary rail are respectively at the first and the second end of the primary rail, while in the open position of the door the second end of the secondary rail is brought closer to the first end of the primary rail and brought farther away from the second end of the primary rail; a first slide unit fastened to the primary rail at its first end and in sliding engagement with the secondary rail, and a second slide unit fastened to the secondary rail at its second end and in sliding engagement with the primary rail, wherein the first slide unit comprises a rolling member in sliding engagement from below on a top horizontal track of the secondary rail facing downwards, and a sliding block engaged on a vertical track of the secondary rail facing the opposite side of the primary rail, and wherein the second slide unit comprises a rolling member in sliding engagement from above on a lower horizontal track of the primary rail facing upwards, and a sliding block engaging on a vertical track of the primary rail facing the opposite side of the secondary track. a primary rail and a secondary rail, placed side by side and extended parallel in a same opening direction for opening the door, wherein the primary rail is fastened to the casing and is extended between a first end and a second end, and the secondary rail is fastened to the door at one of the upper edges thereof and is extended between a first end and a second end, wherein the secondary rail is mounted sliding in relation to the primary rail along said opening direction, in such a way that it can slide between a closed position of the door and an open position of the door, wherein in the closed position of the door the first and the second end of the secondary rail are respectively at the first and the second end of the primary rail, while in the open position of the door the second end of the secondary rail is brought closer to the first end of the primary rail and brought farther away from the second end of the primary rail; a first slide unit fastened to the primary rail at its first end and in sliding engagement with the secondary rail, and a second slide unit fastened to the secondary rail at its second end and in sliding engagement with the primary rail, wherein the first slide unit comprises a rolling member in sliding engagement from below on a top horizontal track of the secondary rail facing downwards, and a sliding block engaged on a vertical track of the secondary rail facing the opposite side of the primary rail, and wherein the second slide unit comprises a rolling member in sliding engagement from above on a lower horizontal track of the primary rail facing upwards, and a sliding block engaging on a vertical track of the primary rail facing the opposite side of the secondary track.