To increase the flow rate of a liquid passing through a reaction bed of a reactor for reacting a liquid flow of petroleum or of a chemical with a treating gas contg. hydrogen in the presence of a catalyst in a countercurrent reaction zone by installing a means for causing at least a part of the treating gas to bypass the reaction zone.

The reactor includes a catalyst bed 4 for causing the reaction between a liquid flow and a treating gas, a nonreaction zone installed in contact with the upstream side of the reaction zone, and a gas-liquid contact zone installed in the nonreaction zone and equipped with an open-structured contact means made of a solid substance. In this reactor, a liquid-introducing means 6 for introducing the unreacted portion of a liquid flow into the reactor is installed at the upper part of the nonreaction zone; a means 8 for leading the unreacted portion of the treating gas to the reactor 1 is installed at the lower part of the reaction zone; a liq. discharge means 10 for discharging the reacted portion of the liquid flow is installed at the lower part of the reaction zone; a means 12 for discharging the reacted portion of the treating gas is installed at the upper part of the nonreaction zone of the reaction zone; and a means 14 for causing the treating gas to bypass the reaction zone is installed along the reaction zone.