The Hidaka metamorphic belt of Hokkaido, Japan is a steeply dipping sequence of metamorphic and igneous rocks, similar to an island-arc type crust, which decrease in metamorphic grade from granulite facies in the west (base) to very low grade in the east (top). This island-arc type crust is thrust over a metamorphosed ophiolitic sequence to the west along the Hidaka Main Thrust. Twenty two biotite and two hornblende samples from metamorphic rocks and tonalites collected from the base to the top of the metamorphic sequence and from various topographic elevations of the Hidaka metamorphic belt yield similar K-Ar ages. These range from 19.1±0.4 Ma to 16.3±0.4 Ma regardless of rock type and metamorphic grade. These ages indicate that the present rock sequence and the steeply eastward dipping structure of the Hidaka metamorphic belt was already formed in or before the late Early Miocene. The steep tilt of the Hidaka island-arc resulted from a dextral thrust movement associated with the collision of the metamorphic belt with the ophiolitic sequence to the west. Their amalgamation, therefore, occurred before the late Early Miocene. A small difference in age of about 1 Ma between hornblende and biotite K-Ar ages indicates rapid uplift rates of 5.8 to 7.8mm/y during the late Early Miocene, using the closure temperatures of 300°C for biotite and 510°C for hornblende, and a geothermal gradient of 30°C/km. The rapid uplift of the metamorphic belt during that time is the result of the dextral transpression associated with the collision. The uplift rates decreased to 0.52 to 0.61mm/y from that time to the present according to the biotite K-Ar ages. Biotite K-Ar ages decrease with increasing topographic elevations of sample localities. This is the reverse of the trend that is usually observed, and suggests differential uplift due to differential movement along the steeply dipping schistosity surfaces within the metamorphic belt.