  We study constraints from perturbativity and vacuum stability as well as the
EWPD in the type II seesaw model. As a result, we can put stringent limits on
the Higgs triplet couplings depending on the cut-off scale. The EWPD tightly
constrain the Higgs triplet mass splitting to be smaller than 40 GeV. Analyzing
the Higgs-to-diphoton rate in the allowed parameter region, we show a possible
enhancement by up to 100 % and 50 % for the cut-off scale of 100 TeV and
$10^{19}$ GeV, respectively, if the doubly charged Higgs boson mass is as low
as 100 GeV.
