Titin is a giant protein that is specifically expressed in striated muscle and essential for the maintenance of sarcomere structure and function. Recently, the N-terminal fragment of the Titin (N-titin) has been reported to show high levels in human urine in patients with muscular diseases and is expected to serve as a diagnostic biomarker for these diseases. In this study, we examined the utility of N-titin as a biomarker to detect muscle atrophy in mice. Male BALB/c mice (6 weeks of age, n=5 per group) were given 10 mg/L dexamethasone (DEX) dissolved in drinking water for 4 weeks. The gastrocnemius muscle (GAS) weight was significantly decreased and mRNA levels of muscle atrophy-related genes (Atrogin-1 and MuRF-1) were increased in the GAS after 4 weeks of DEX treatment. Although there were no degenerative/necrotic changes in the histopathological examination, the muscle fiber cross-sectional area significantly decreased in the GAS. On the other hand, there were no DEX treatment-related changes in the muscle weights and the muscle fiber cross-sectional area in the soleus muscle. These results suggest that 4-week of DEX treatment preferentially caused atrophy of fast-dominant muscle. Under the condition of this study, urinary N-titin/CRN ratio markedly increased from Week 2 of the DEX treatment. From the above results, the urinary N-titin/CRN ratio could be a biomarker for monitoring skeletal muscle atrophy in mice.