A cylinder lock has a housing with a rotatably insertable cylinder having a key passage for an insertable flat key whose front part has locking notches associated with cylinder pins arranged in openings of the cylinder for retaining the cylinder pins. The flat key is provided at both its lateral sides with longitudinal grooves for receiving longitudinal ribs provided on the lateral sides of the key passage for the lateral profile variation. The longitudinal grooves on both lateral faces of the flat key lie with their bottom faces on lines which extend parallel to and at a distance from one another. The parallel lines extending at a distance from one another are arcuate at least in one partial region, and the convexly protruding central part of one arcuate line extend at least close to a straight line connecting the end points of the second arcuate line.