Not to nitpick, but that's 480,000 households, not viewers. Presumably the average number of viewers per households is greater than one, though given Japan's ageing population, household sizes are shrinking over time.
From the
Japanese Statistics Bureau:
"The Population Census shows that Japan had 48.22 million households in 2005. Of that total, 58.9 percent were nuclear-family households, and 27.6 percent were one-person households.
"From the 1920s to the mid-1950s, the average number of household members remained about five. However, reflecting the progressive decline in the birth rate through the 1960s, the size of household was down significantly in 1970, to 3.41 members. The size of household members continued to decline to 2.60 in 2005, principally due to the increase of one-person households and the conversion of households into nuclear families."
So a 1% household rating might translate into something like a million viewers.