  Even after elaborate investigations for 30 years, we still do not know well
how the progenitor of SN 1987A has evolved. To explain unusual red-to-blue
evolution, previous studies suggest that in a red giant stage either the
increase of surface He abundance or the envelope mass was necessary. It is
usually supposed that the He enhancement is caused by the rotational mixing,
and the mass increase is by a binary merger. Thus, we have investigated these
scenarios thoroughly. The obtained findings are that rotating single star
models do not satisfy all the observational constraints and that the
enhancement of envelope mass alone does not explain observations. Here, we
consider a slow merger scenario in which both the He abundance and the envelope
mass enhancements are expected to occur. We indeed show that most observational
constraints such as the red-to-blue evolution, lifetime, total mass, position
in the HR diagram at collapse, and the chemical anomalies are well reproduced
by the merger model of 14 and 9 M$_{\odot}$ stars. We also discuss the effects
of the added envelope spin in the merger scenarios.
