  We revisit the minimal Nelson-Barr model for solving the strong CP problem
through the idea of spontaneous CP breaking. The minimal model suffers from the
quality problem, which means that the strong CP angle is generated by
higher-dimensional operators and one-loop effects. Consequently, it has been
considered that there is a cosmological domain wall problem and that
leptogenesis does not work. We point out that just imposing an additional
approximate global symmetry solves the quality problem. We also propose a
simple solution to the domain wall problem and show that the thermal
leptogenesis scenario works.
