  The study of QCD phase diagram is very interesting, but we have never
understood it well. This is because we face a problem at finite density in QCD.
The problem is called sign problem. It causes a decrease of the calculation
accuracy. This is why, we can not calculate physical quantities accurately at
finite chemical potential. In this study, we try to beat the sign problem using
canonical approach of finite density lattice QCD. Although it is known that the
canonical approach has several numerical problems, we can reduce them and
calculate thermodynamic observables accurately at finite density. In this
thesis, we will see how to improve the canonical approach and a result of
thermodynamic observables which are related to the QCD phase transition at
finite density. Our study focused on baryon number susceptibility. A peak in
baryon number susceptibility corresponds to the confinement--deconfinement
phase transition. In this study, we do not see the QCD phase transition yet.
However, in this thesis, we find that canonical approach can explore the QCD
phase diagram beyond mu_B/T = 3 (mu_B is the baryon chemical potential). That
means, we have explored the QCD phase structure beyond the validity range of
Taylor expansion and reweighting method. With our improvement, canonical
approach has obtained the possibility for investigation of thermodynamic
observables at any chemical potential.
