  We study radiative corrections to the mass spectrum and the triple Higgs
boson coupling in the model with an additional Y=1 triplet field. In this
model, the vacuum expectation value for the triplet field is strongly
constrained from the electroweak precision data, under which characteristic
mass spectrum appear at the tree level; i.e., $m_{H^{++}}^2-m_{H^+}^2\simeq
m_{H^+}^2-m_A^2$ and $m_A^2\simeq m_H^2$, where the CP-even ($H$), the CP-odd
($A$) and the doubly-charged ($H^{\pm\pm}$) as well as the singly-charged
($H^\pm$) Higgs bosons are the triplet-like. We evaluate how the tree-level
formulae are modified at the one-loop level. The $hhh$ coupling for the
standard model-like Higgs boson ($h$) is also calculated at the one-loop level.
One-loop corrections to these quantities can be large enough for identification
of the model by future precision data at the LHC or the International Linear
Collider.
