We developed the glucose-responsive florescent tetra-PEG gel based on diboronic acid for implantable continuous glucose monitoring. The fluorescent dye having diamine unit was immobilized into tetra-PEG gel formed by mixture of tetraamine-terminated-PEG (TAPEG) and tetra-NHS-glutarate-terminated-PEG (TNPEG). When the glucose concentration increased from 0 mg•dL−1 to 1,000 mg•dL−1, the fluorescence intensity of the gel also become higher. In vivo glucose monitoring was conducted using implantable wireless fluorescent monitoring device equipped with the tetra-PEG gel in the back of rat. We have successfully detected the glucose concertation of interstitial fluid continuously. By combining the tetra-PEG gel with small and implantable florescent monitoring device, continuous glucose monitoring (CGM) system using fluorescent hydrogel can be a powerful approach for practical in vivo.