  A phase structure and phase transitions are investigated in the modified Bonn
quark model. The force strength, which represents the particle-hole type
interaction, is devised so as to depend on the quark number, namely quark
number density. As a result, it is shown that the phase transition from the
quark-triplet state as a nucleon to the quark-pair state as a
color-superconducting state occurs as the density increases.
