The invention relates to a device for defining a cutting plane for bone resection such that it can be adjusted, comprising a base (5) that can be positioned on the bone and a cut guiding element (8) which is placed on the base (5) such that its position can be changed and which comprises a cut guiding structure (34) for guiding a cutting tool in the cutting plane, the cut guiding base (8) being tiltable relative to the base (5) about at least one first axis by means of a first adjustable tilt mechanism to adjust the angle of the cutting plane. The aim of the invention is to develop said device such that the tilt mechanism is easy to handle and reliable with respect to the position fidelity of the cutting plane once it is adjusted also when the resection cut is carried out. According to the invention, the tilt mechanism comprises a rectilinear guide (24, 29) that is arranged on a first element of the device and a slide (25, 30) that can be linearly displaced along the guide (24, 29) in the longitudinal direction thereof. The tilt mechanism, on a second element of the device, which can be tilted about the first axis relative to the first element, comprises a guide portion relative to which the an effective portion of the slide (25, 30) is displaced when the slide is displaced, the guide portion and the effective portion having interacting structures (26, 31, 32, 33) that form a sliding guide effective at a right angle to the longitudinal direction.
