  We study the effect of massive isocurvaton on density perturbations in
quasi-single field inflation models, when the mass of the isocurvaton M becomes
larger than the order of the Hubble parameter H. We analytically compute the
correction to the power spectrum, leading order in coupling but exact for all
values of mass. This verifies the previous numerical results for the range
0<M<3H/2 and shows that, in the large mass limit, the correction is of order
H^2/M^2.
