  We describe a new method to compute renormalized coupling constants in a
Monte Carlo renormalization group calculation. The method can be used for a
general class of models, e.g., lattice spin or gauge models. The basic idea is
to simulate a joint system of block spins and canonical demons. In contrast to
the Microcanonical Renormalization Group invented by Creutz et al. our method
does not suffer from systematical errors stemming from a simultaneous use of
two different ensembles. We present numerical results for the $O(3)$ nonlinear
$\sigma$-model.
