Although arsenic (As) is a well-known toxic element, arsenic trioxide (ATO) is an effective pharmaceutical agent for treating acute promyelocytic leukemia (APL). This study aims to determine the total arsenic concentrations in urine of an APL patient using a low-cost simple and sensitive method. Total arsenic concentrations in urine of a pediatric patient with APL treated with ATO during induction therapy were monitored by hydride generation-microwave plasma-atomic emission spectrometry (HG-MP-AES). The limit of detection and limit of quantification were calculated to be 0.32 and 1.09 ng/mL, respectively. Accuracy and precision were good. The total urinary arsenic concentrations were equivalent to those reported in previous studies. During consolidation therapy, the total urinary arsenic concentrations increased steadily for the first 11 days, and then increased with fluctuations. Our results show that HG-MP-AES is a low-cost, operationally simple, and high-sensitivity method suitable for monitoring total urinary arsenic concentrations in APL patients subsequent to ATO treatment. Our method could contribute to determining the dosage of ATO and reducing its adverse effects.