Spectrophotometric observations in the optical and near-infrared of six giant H II regions in the high metallicity spiral galaxy M51 are presented. Five of the regions are located in the spiral arms, and the sixth one is very close to the nucleus. These observations and the application of adequate photo-ionization models allow the determination of the functional parameters of the regions: ionization parameter, effective temperature of the ionizing radiation, and metallicity. The results of this work confirm previously found trends of decreasing ionization parameter and radiation hardness with increasing metallicity. N/O and S/O ratios are in general higher and lower than solar, respectively. The first trend is expected from the viewpoint of stellar nucleosynthesis and chemical evolution theories. However, the existence of a lower S/O ratio in regions of high metallicity, if confirmed as real, needs to be accounted for.