We report our experimental studies on (p, 2p) reactions from two kinds of view points, namely to investigate single particle properties of nuclei and to observe possible modification of NN interactions in nuclear field. Comparison of the cross section data with theoretical predictions and data of (e, e′p) reaction suggests that the reaction mechanizsm at incident energies of around 400 MeV is simple enough to extract spectroscopic information through the (p, 2p) reactions. In addition, some analyzing power data show anomaly which is not seen in cross section data and, therefore, which suggest importance of polarization studies. On the examination of the NN interaction, systematic study on analyzing powers for 1s1/2-proton knockout has been continued. A new systematics, a Q-value dependence the analyzing power was newly observed.