The present invention provides a rock bit which enables a reduction in surface pressure from a well wall and which allows possible vibration to be suppressed. A first of the present invention provides a rock bit including a bit body with a journal portion, a cone portion configured to be rotatable with respect to the bit body, a bearing portion located between the cone portion and the journal portion, and a seal provided between the cone portion and the journal portion to seal the bearing portion, wherein the cone portion includes a gauge surface that contacts a side wall of a well, and a diamond composite material is preferably disposed extensively and smoothly on the gauge surface. In a second aspect of the present invention, a ring bit is provided at an outer peripheral portion of a bit body. Furthermore, a cone portion of a tri-cone bit includes no gauge tip or surface chip that contacts the side wall of the well but only cemented carbide tips that contact a bottom of the well.