The milestone took the 3DS nine weeks longer to reach than it took for the original DS.
The Nintendo 3DS has sold one million units in Japan thirteen weeks after its release according to the May sales report of Japanese game magazine Enterbrain.
The 3DS has been experiencing slower sales than Nintendo had hoped since it launched in Japan February 26. For reference, the original Nintendo DS sold one million units in that market in four weeks back in 2004.
It is important to note that the Nintendo 3DS is considerably more expensive (25,000 Yen versus 15,000) than its predecessor, and came out in February whereas the Nintendo DS launch in December.