PURPOSE: To maintain each hook level of two crabs in horizontality with yet more certainty by discriminating a difference in these hook levels from a speed difference between first hoisting drum revolution and second hoisting drum revolution of both these crabs, and controlling inverter controlling frequency.

CONSTITUTION: Each signal of both first and second synchro transmitters 5, 6 installed in each turning shaft of respective hoisting drums 1, 2 is received by a differential receiver 7, finding a speed difference between both these hoisting drums 1, 2, and a difference in both first and second hook levels is measured. For example, when a hook level of a first crab is increased, command frequency to a first electric motor M1 is lowered, and lifting speed of a hook of the first crab is decelerated. In contrast with this, when the hook level of the first crab is decreased, the command frequency to the first motor M1 is raised up to some extent, and the lifting speed of the hook of the first crab is increased. Thus, the hook levels of both crabs are equalized, whereby a long material such as a vehicle or the like is maintained in horizontality and thus combined lifting crane operation is carried out.