The nuclear magnetic resonance of Co-59 nuclei in magnetic domains of ErCo5 has been measured under external fields up to 50 kOe at 4.2 K. To assign the observed NMR signals to each Co site, the Co-59 nuclear magnetic resonance of Er(Co1-xNi(x))5 has also been measured under the same condition to the above.From the NMR study of ErCo5, we have found that the cobalt hyperfine field at the 2c site has the positive sign in contrast to the negative sign at the 3g site. This suggests that the cobalt atoms at the 2c sites in ErCo5 have large orbital moment.