  The gravitational interaction is ubiquitous and the effect of gravitational
particle production necessarily contributes to the dark matter abundance. A
simple candidate of dark matter is a scalar particle, whose only renormalizable
interaction is the Higgs portal coupling. We show that the abundance of Higgs
portal dark matter is significantly affected by the gravitational production
effect. In particular, the gravitational production from the coherently
oscillating inflaton field during the reheating often gives dominant
contribution.
