  We reexamine inflation due to a constrained inflaton in the model of a
complex scalar. Inflaton evolves along a spiral-like valley of special scalar
potential in the scalar field space just like single field inflation.
Sub-Planckian inflaton can induce sufficient $e$-foldings because of a long
slow-roll path. In a special limit, the scalar spectral index and the
tensor-to-scalar ratio has equivalent expressions to the inflation with
monomial potential $\varphi^n$. The favorable values for them could be obtained
by varying parameters in the potential. This model could be embedded in a
certain radiative neutrino mass model.
