As part of a series of studies on the synthesis of new antibacterial agents, tetracyclic compounds having a furo [2, 3-g] quinoline moiety as a common structural unit were synthesized, and their antibacterial activities were examined. Among them, 3-methyl-7-oxo-2, 3-dihydro-7H-furo [2, 3-h] pyrido [1, 2, 3-de] [1, 4] benzoxazine-6-carboxylic acid (4c) exhibited the most potent antibacterial activity against Gram-positive and-negative organisms, including Pseudomonas aeruginosa, and it showed low acute toxicity to mice.