Improvement in circuit performance of Josephson logic devices is described and discussed. A high sensitivity switching gate of asymmetrical two junction interferometer is proposed, in which the Josephson threshold current of the junctions is unequal to each other but the bias feeding point is selected so as to balance inductance threshold current product on the left and the right hand side branch. Transmission lines with characteristic impedance as high as 4Ω were realized on a new superconducting ground plane structure with SiO2 overlying insulation. High resistivity resistor material of gold-indium alloy was also developed for resistive termination. A series-connected 4 stage OR gate chain was experimentally constructed with the present switching gates, transmission lines, and termination resistors. The circuit performance measurements demonstrated that the average speed power product was as small as 0.9 fJ per single stage.