  We propose to use proton knockout reactions (p,2p) from a deeply bound orbit
as a new probe into three-nucleon-force (3NF) effects. The remarkable advantage
of using (p,2p) reaction is that we can choose an appropriate kinematical
condition to probe the 3NF effects. We analyze (p,2p) reactions on a 40Ca
target within the framework of distorted-wave impulse approximation with a
g-matrix interaction based on chiral two- and three-nucleon forces. The chiral
3NF effects significantly change the peak height of the triple differential
cross section of (p,2p) reaction. We also clarify the correspondence between
the (p,2p) cross sections and the in- medium pp cross sections.
