We report our recent project to study the QCD phase diagram by the canonical approach (CA), which is expected to solve the sign problem. First we briefly describe the sign problem and several approaches to fight against it. Then we argue that the CA may be a break-through if we combine it with the multiple precision calculation. We study the method using the hopping parameter expansion (HPE), in order to investigate how the CA works. We show explicitly how we calculate the winding number in HPE and obtain the fugacity expansion whose coefficients are the canonical partition functions. Finally we discuss dark sides of the current CA, which should be beaten for the method to become a real tool of QCD phase explorers.