  Three-dimensional black holes coupled to a self-interacting scalar field is
considered. It is known that its statistical entropy $a' la$ Strominger does
$not$ agree with the Bekenstein-Hawking (BH) entropy. However I show that, by a
careful treatment of the vacuum state in the {\it canonical} ensemble with a
fixed temperature, which is the same as that of the BTZ black hole without the
scalar field, the BH entropy is exactly produced by the Cardy's formula. I
discuss its several implications, including the fate of black holes,
no-scalar-hair theorems, stability, mirror black holes, and one-loop
corrections.
