An X-ray tube, comprising: a cathode to emit electrons; an anode target to emit X-rays by impingement of the electrons emitted from the cathode; a vacuum envelope housing the cathode and the anode target, and a cooling system forming a cooling passage through which coolant is to flow, wherein heat released from the anode target is conducted to the cooling system; characterized in that the vacuum envelope has a first metal member connected to the anode target and extending in a direction along a tube axis of the x-ray tube, and having an X ray radiation window; a second metal member connected to the first metal member, extending in a direction along the tube axis, and having a coefficient of thermal expansion lower than that of the first metal member; and an electrically insulating annular ceramic member connected to the second metal member and the cathode, and projecting from the cathode in a direction perpendicular to the tube axis; and the X-ray tube further comprises an adapter placed so as to be in contact with the first metal member and to surround the second metal member, wherein the adapter has a thermal conductivity higher than that of the second metal member; and a heat-transfer medium placed between the ceramic member and the adapter; and wherein the cooling system is connected to the first metal member such that heat released from the anode target is conducted to the cooling system directly or indirectly via the first metal member, and heat released from the cathode is conducted to the cooling system indirectly via the ceramic member, the heat-transfer medium, the adapter and the first metal member. a cathode to emit electrons; an anode target to emit X-rays by impingement of the electrons emitted from the cathode; a vacuum envelope housing the cathode and the anode target, and a cooling system forming a cooling passage through which coolant is to flow, wherein heat released from the anode target is conducted to the cooling system; characterized in that the vacuum envelope has a first metal member connected to the anode target and extending in a direction along a tube axis of the x-ray tube, and having an X ray radiation window; a second metal member connected to the first metal member, extending in a direction along the tube axis, and having a coefficient of thermal expansion lower than that of the first metal member; and an electrically insulating annular ceramic member connected to the second metal member and the cathode, and projecting from the cathode in a direction perpendicular to the tube axis; and the X-ray tube further comprises an adapter placed so as to be in contact with the first metal member and to surround the second metal member, wherein the adapter has a thermal conductivity higher than that of the second metal member; and a heat-transfer medium placed between the ceramic member and the adapter; and wherein the cooling system is connected to the first metal member such that heat released from the anode target is conducted to the cooling system directly or indirectly via the first metal member, and heat released from the cathode is conducted to the cooling system indirectly via the ceramic member, the heat-transfer medium, the adapter and the first metal member.