A drive power transmission device is disclosed having a drive mechanism for transmitting a drive power between first and second rotary members rotatable relative to each other, by the operation of an electromagnetic type clutch mechanism which brings a friction clutch into connection. A magnetic path generated around an electromagnet of the electromagnetic type clutch mechanism includes a clutch magnetic path in which a magnetic flux passes to reciprocate plural times across th friction clutch, and diamond-like carbon surface treatment is given on a part or all of the friction contact surfaces of the friction clutch to form a hard amorphous carbon film thereon. Thus, the transmission device can be constructed using a smaller number of the outer and inner plates for the electromagnetic type clutch mechanism, so that it can take a small construction with a longer durability life.