A drill bit of a diamond-coated sintered body has a substrate mainly composed of Si.sub.3 N.sub.4 or SiC. At least a wear portion of the bit is coated with a diamond layer. If the bit is of the throw-away kind, an insert is removably secured to a shank. At least the insert or a wear portion thereof is coated with a diamond layer. The sintered body contains a prescribed amount of a sintering assistant or aid selected from MgO, Y.sub.2 O.sub.3 and the like. The diamond coating layer has a thickness of at least 0.1 .mu.m.