Background: After changing the urinary protein assay method from the pyrogallol red (PR) method to the benzethonium chloride (BC) method in 2019, the number of patients with a urinary protein-to-creatinine ratio (PCR) of 0.15 g/gCr or more increased. This study aimed to investigate the possible causes behind this increase. Methods: The subjects were elementary and junior high school students who had undergone urinalysis in our hospital at least once a year for 3 consecutive years from April 2017 and whose diagnoses were no abnormality or asymptomatic hematuria. The urinary PCR of the earliest morning samples were analyzed. Results: The participants were 74 patients (22 men and 52 women) with a median age of 12.3 years in 2019. The median urinary PCR was higher in 2019 using the BC method (0.14 g/gCr) than in 2017 and 2018 using the PR method (0.09 and 0.08 g/gCr, respectively). Conclusion: When the BC method was used to assess urinary protein, the PCR tended to be higher than that in the PR method. When evaluating the PCR, the urinary protein assay used should be noted.