To secure an effective cooling method in the drilling part of a rotary drilling bit and also to make improvements in service life and drilling speed.

This rotary drilling bit's drilling part cooling method is so constituted that a communicating hole 1 ranging from a rear end to the tip part is installed in the inner part of a rotary drilling bit, and a coolant is made to flow in from the rear end, or pressed-in, and then it is made to flow out of an optional position of the rotary drilling bit, or sprayed out, thereby cooling a drilling part of this rotary drilling bit. Likewise, this drilling bit is attached to a tip of a wet-type drill system on the market, and it pours the coolant from a cooling water exhaust nozzle of this wet-type drill system.