Sustained drug-releasing systems have the potential to enhance beneficial effects and reduce side effects of various drugs. Injectable polymer (IP) systems forming hydrogel in situ are useful materials for preparing sustained drug-releasing systems. We previously reported the biodegradable IP systems using triblock copolymers of poly(ϵ-caprolactone-co-glycolic acid) (PCGA) and poly(ethylene glycol) (PEG) (PCGA-b-PEG-b-PCGA, tri-PCG). The IP hydrogel systems are expected to be applied as sustained drug-releasing depots because the solution exhibits an immediate sol-to-gel transition upon heating to body temperature. However, sustained release of water-soluble low-molecular-weight drugs was difficult because the sizes of such drugs are much smaller than the hydrogel network. To solve this problem, we focused on liposomes that have been studied as carriers for various drugs. In this study, we investigated the drug-releasing profile from tri-PCG IP hydrogel containing model drug-loaded liposomes. In vitro release rate of pyranine from IP hydrogel containing pyranine-loaded liposome was slower than IP hydrogel entrapping intact pyranine. These results suggested that the liposome stably retained pyranine after gelation of tri-PCG solution, and diffusion of pyranine inside of the hydrogel was suppressed because of enlargement of apparent size. The IP formulations were subcutaneously injected into the backs of mice. Groups treated with IP hydrogel containing pyranine-loaded liposome maintained the plasma concentration of pyranine for 30 days. We succeeded in demonstrating a sustained releasing system for water-soluble low-molecular-weight drugs using IP gel and drug-loaded liposomes.