The effects of the horizontal connection length (0.3 less than or equal to L-c less than or equal to 0.6 m), the cross-sectional area ratio of downcomer-to-riser (0.28 less than or equal to A(d)/A(r) less than or equal to 0.53), and the superficial gas velocity on the circulation gas flow rate were studied. The circulation liquid velocity, the downcomer gas holdup, and the bubble size in the downcomer were also investigated. In order to measure the circulation gas flow rate, a gas-liquid separator was installed at the bottom connection between riser and downcomer. Both of the circulation gas flow rate and the circulation liquid velocity strongly depended on the superficial gas velocity as well as the cross-sectional area ratio of downcomer-to-riser and the horizontal connection length. An useful correlation for the circulation gas flow rate was obtained in terms of the superficial gas velocity, the cross-sectional area ratio of downcomer-to-riser and the horizontal connection length.