We determined the K -Ar ages for the high -magnesian andesites from Miocene -Pliocene period from Hikosan and Nagasaki areas in northern Kyushu, Japan. The obtained K -Ar ages of Hikosan and Nagasaki samples are 4.0 -3.6 Ma and 5.8 -3.7 Ma, respectively. These K -Ar ages suggest that the magmatisms of the high -magnesian andesites coincided with the development of the Okinawa Trough. The upwelling of the hot asthenospheric mantle associated with the backarc rifting probably has a close relationship with the origin of the northern -Kyushu high -magnesian andesite magma.