  Powerful radio galaxies at high redshift are highly useful in studies of
early evolution of AGN-hosting galaxies because their observed optical and near
infrared light are dominated by their stellar population rather than the
nonthermal continuum emitted by the central engine of AGNs. In addition, the
presence of AGN activity in them implies that a supermassive black hole has
been already made in their nuclei. These properties allow us to investigate a
possible starburst-AGN connection in early universe and then provide some
crucial hints for the formation mechanism of supermassive black holes. Taking
observational properties of high-z powerful radio galaxies into account, we
discuss a possible formation mechanism of supermassive black holes in their
nuclei.
