The causes of premature loss of deciduous teeth have been reported to be associated with caries, periodontal disease, trauma, occlusal trauma, bruxism, and systemic diseases. However, in daily clinical practice, there are rare cases in which it is difficult to identify the cause of premature loss. Here, we report a case of premature loss of a deciduous canine in a 4-year and 6-month-old boy.The patient was referred to our clinic with premature loss of the mandibular left deciduous canine. As the intraoral findings, the mandibular left deciduous canine tooth had fallen out, and the dropout cavity was observed. He had tooth attrition caused by bruxism, but no dental history of caries or trauma was noted. To begin with, a blood test was performed for screening of systemic diseases. In addition, bacteriological examination was performed to evaluate the association with periodontopathic bacteria. However, there were no abnormal findings in those examinations. Next, we carefully observed the fallenout tooth. Although there was no abnormal appearance on visual inspection, fracture lines with a crack degree were found under stereoscopic microscope observation. Furthermore, CT examination revealed that the fracture continued from the crown surface to the pulp cavity. Finally, excessive physical irritation caused by bruxism had resulted in crown fracture, which might induce pulp necrosis and apical periodontitis. Furthermore, excessive occlusal pressure due to the bruxism and lateral movement would affect this condition. Because primary teeth have less mature calcification than permanent teeth and due to their structural characteristics, tooth microcracks often occur. Furthermore, excessive occlusal force applied to the tooth due to trauma or bruxism may cause microcracks, which may lead to unexpected crown fractures and premature loss of teeth.