Locking device for a flap or a door, comprising a substantially flat support structure, a handle element for unlocking the locking device; and a latch with a hook; wherein: the support structure can be non-positively connected to the flap and has - a pivot bearing for a rotation bolt; and - at least one cylindrical straight-guiding plain bearing bush; the handle element has - a handle recess and at least one plain bearing bolt and - provides a receptacle for an activation bolt arranged in a fixed position in the handle element and aligned at right angles to the plain bearing bolt; the latch has - a slotted link designed as an elongated hole or groove for guiding the activation bolt; wherein, in a pre-assembled state, - the plain bearing bolt of the handle element is guided in a longitudinally displaceable manner in the plain bearing bush of the support structure; - the latch is rotatably mounted in the pivot bearing of the support structure about the rotation bolt between a "locked" and "open" position; and - the activation bolt is in engagement with the slotted link, characterized in that the guidance of the handle element is carried out in a straight line during opening and closing by the combination of plain bearing bolt /plain bearing bush and this straight-line movement is translated by the co-moved activation bolt via the slotted link into a rotary movement of the latch, which causes the unlocking of the locking device. the support structure can be non-positively connected to the flap and has - a pivot bearing for a rotation bolt; and - at least one cylindrical straight-guiding plain bearing bush; - a pivot bearing for a rotation bolt; and - at least one cylindrical straight-guiding plain bearing bush; the handle element has - a handle recess and at least one plain bearing bolt and - provides a receptacle for an activation bolt arranged in a fixed position in the handle element and aligned at right angles to the plain bearing bolt; - a handle recess and at least one plain bearing bolt and - provides a receptacle for an activation bolt arranged in a fixed position in the handle element and aligned at right angles to the plain bearing bolt; the latch has - a slotted link designed as an elongated hole or groove for guiding the activation bolt; - a slotted link designed as an elongated hole or groove for guiding the activation bolt; wherein, in a pre-assembled state, - the plain bearing bolt of the handle element is guided in a longitudinally displaceable manner in the plain bearing bush of the support structure; - the latch is rotatably mounted in the pivot bearing of the support structure about the rotation bolt between a "locked" and "open" position; and - the activation bolt is in engagement with the slotted link, - the plain bearing bolt of the handle element is guided in a longitudinally displaceable manner in the plain bearing bush of the support structure; - the latch is rotatably mounted in the pivot bearing of the support structure about the rotation bolt between a "locked" and "open" position; and - the activation bolt is in engagement with the slotted link, characterized in that the guidance of the handle element is carried out in a straight line during opening and closing by the combination of plain bearing bolt /plain bearing bush and this straight-line movement is translated by the co-moved activation bolt via the slotted link into a rotary movement of the latch, which causes the unlocking of the locking device.