An apparatus for checking the diameter of crankpins (18) of a crankshaft (34) in the course of the machining in a grinding machine comprises a first arm (9) rotating with respect to a support (5) arranged on the grinding-wheel slide (1) of the grinding machine, a second arm (12) rotating with respect to the first, a reference device (20) carried by the second arm and a measuring device (16, 17, 40-45) associated with a reference device. A guide device (21), fixed to the reference device (20), enables the apparatus to engage a crankpin, in the course of the orbital motion of the crankpin, and limit the displacements of the first arm and those of the second arm when a control device (28-30) displaces the apparatus to a rest position.