In a surgical instrument for tensioning two plate-shaped engaging elements with respect to each other at opposite sides of bone portions, a first engaging element carrying next to each other at a spacing from each other two pin-shaped or rod-shaped connecting members which pass through openings in the second engaging element, with a supporting surface for engagement on the side of the second engaging element that faces away from the first engaging element, and with a tensioning device engaging the connecting members for displacement of the connecting members and consequently of the first engaging element in the direction towards the second engaging element, to enable an individual adjustment of the engaging elements to the shape of the bone portions, it is proposed that the surgical instrument comprise next to each other two grip elements, each releasably connectable to a connecting member, and that the tensioning device engage the grip elements through a compensating element which, upon tensioning of the tensioning device, allows a differently sized displacement of the two grip elements and consequently of the two connecting members.