  Primordial black holes can be formed from the collapse of large-amplitude
perturbation on small scales in the early Universe. Such an enhanced spectrum
can be realized by introducing a flat region in the potential of single-field
inflation, which makes the inflaton go into a temporary ultraslow-roll period.
In this paper, we calculate the bispectrum of curvature perturbation in such a
scenario. We explicitly confirm that bispectrum satisfies Maldacena's theorem.
At the end of the ultraslow-roll period, the bispectrum is generated by bulk
interaction and field redefinition. At the end of inflation, bispectrum is
generated only by bulk interaction. We also calculate the one-loop correction
to the power spectrum from the bispectrum, called the source method. We find it
consistent with the calculation of the one-loop correction from the
second-order expansion of in-in perturbation theory.
