The need for accurate and efficient antibiotic susceptibility testing (AST) has been emphasized with respect to the emerging antimicrobial resistance of pathogenic bacteria which has increased over the recent decades. In this study, we introduce a microfluidic system that enables rapid formation of the antibiotic concentration gradient with convenient bacterial growth measurement based on color scales. Furthermore, we expanded the developed system to analyze combinatory effects of antibiotics and measured the collective antibiotic susceptibility of bacteria compared to single microfluidic AST methods. By injecting a continuous flow precisely into the channel, the system enabled the concentration gradient to be established between two parallel channels of different antibiotic concentrations within 30 min, before bacteria enter the exponential growth phase. Moreover, the local bacterial growth levels under antibiotic gradient were quantitatively determined by calculating the position-specific grayscale values from the microscopic images and were compared with the conventional optical density measurement method. We tested five antibiotic types on our platform for the pathogenic Gram-negative bacteria strain Pseudomonas aeruginosa, and we were able to determine the minimum inhibitory concentration (MIC) at which 90% to 95% of bacterial growth was inhibited. Finally, we demonstrated the efficacy of our system by showing that most of the antibiotic MICs determined in our platform show good agreement with the MIC range suggested by the Clinical and Laboratory Standards Institutes.