  The Fast Blue Optical Transient (FBOT) ATLAS18qqn (AT2018cow) has a light
curve as bright as superluminous supernovae but rises and falls much faster. We
model this light curve by circumstellar interaction of a pulsational
pair-instability (PPI) supernova (SN) model based on our PPISN models studied
in previous work. We focus on the 42 $M_\odot$ He star (core of a 80
$M_{\odot}$ star) which has circumstellar matter of mass 0.50 $M_\odot$. With
the parameterized mass cut and the kinetic energy of explosion $E$, we perform
hydrodynamical calculations of nucleosynthesis and optical light curves of
PPISN models. The optical light curve of the first $\sim$ 20 days of AT2018cow
is well-reproduced by the shock heating of circumstellar matter for the $42
~M_{\odot}$ He star with $E = 5 \times 10^{51}$ erg. After day 20, the light
curve is reproduced by the radioactive decay of 0.6 $M_\odot$ $^{56}$Co, which
is a decay product of $^{56}$Ni in the explosion. We also examine how the light
curve shape depends on the various model parameters, such as CSM structure and
composition. We also discuss (1) other possible energy sources and their
constraints, (2) origin of observed high-energy radiation, and (3) how our
result depends on the radiative transfer codes. Based on our successful model
for AT2018cow and the model for SLSN with the CSM mass as large as $20
~M_\odot)$, we propose the working hypothesis that PPISN produces SLSNe if CSM
is massive enough and FBOTs if CSM is less than $\sim 1 ~M_\odot$.
