The earliest onset of the Asian summer monsoon rainfall occurs in May over Indochina and its vicinity. While heating over the Tibetan Plateau has been considered as a major driving mechanism to induce the onset, several other possible mechanisms have been proposed in recent years. This study explores the monsoon onset over Indochina through several integrations of a global atmospheric model with various boundary conditions in topography and sea surface temperature (SST). Results indicate that warm SST which appears to migrate from the ocean around Indochina to the Western Pacific during the rainy season plays a major role to induce the monsoon over Indochina. The onset requires the warm SST around Indochina as a part of the asymmetric SST field of the globe. The Tibetan Plateau is not an major factor for the onset. The recognized earliest onset can thus be interpreted as the onset of marine monsoon.