Monitoring of proteins on biosensor surfaces is required for developing high performance biosensors. In this study, distributions of proteins, protein A and immunogloblin G (IgG), immobilized on glass plates were monitored by means of a new protein monitoring technique using time-of-flight secondary ion mass spectrometry (TOF-SIMS) that provides specific chemical imaging. The TOF-SIMS spectra of the protein immobilized on glass plates were analyzed by means of mutual information to select the appropriate fragment ions related to each protein for chemical imaging of proteins. Finally the distributions of protein A and IgG are obtained respectively with this method.