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.
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