The efficiency ratio of massive star formation in the arms of the spiral NGC 628 to that in the interarm region has been determined from the sizes and positions of its H II regions following the method described in Cepa and Beckman (1989). The variation of this relative star formation efficiency with radius shows a very similar behaviour for each of the three arms of this galaxy, with a minimum at 100 arc sec from the center, which can be identified with the corotation radius assuming a pattern speed of 56 km/s kpc. The results support the hypothesis that the spiral arms of NGC 628 are driven by a density wave which triggers the star formation according to the spiral density wave theory.