From Wikipedia:
// Immigration
Since the Japanese population census asks the people's nationality rather than their ethnic background, naturalized Japanese citizens and Japanese nationals with multi-ethnic background are considered to be ethnically Japanese in the population census of Japan. Thus, in spite of the widespread belief that Japan is ethnically homogeneous, it is probably more accurate to describe it as a multiethnic society.
According to the Japanese immigration centre, the number of foreign residents in Japan has steadily increased, and the number of foreign residents (excluding illegal immigrants and short-term visitors such as foreign nationals staying less than 90 days in Japan[9]) were more than 2.2 million people in 2008.
Among the immigrants, Japan accepts a steady flow of 15,000 new Japanese citizens by naturalization (帰化) per year.[10] Indeed, the concept of the ethnic groups by the Japanese statistics is different from the ethnicity census of North American or some Western European statistics. For example, the United Kingdom Census asks ethnic or racial background which composites the population of the United Kingdom, regardless of their nationalities.[11] The Japanese Statistics Bureau, however, does not have this question yet.
In the 1990s and early 2000s, Japanese diplomats signed agreements with South Asian officials to obtain an estimated 50,000 temporary "guest workers" to work in Japan. Similar guest-worker agreements with Latin American countries, such as Brazil, Uruguay, Chile, Mexico and Peru have brought another 20,000 foreigners to Japan, including Latin Americans of Japanese descent who might culturally assimilate into the Japanese population. //