Monster Hunter is a definite contender for the most popular game series in Japan. It's practically sold Sony's PlayStation Portable single-handedly over the past four years, racking up 6 million sales on that platform alone. It wasn't exactly unpopular on the PS2 either; Monster Hunter games have sold over a million there too.
Altogether, assuming every single copy of a Monster Hunter game were sold to a different person, that would be five and a half percent of the entire population of Japan. Monster Hunter Tri, the latest entry in the series, has sold over a million copies on Wii in Japan already, and it on its way to the rest of the world this month.