Internal ionizing radiation exposure dose is estimated indirectly by bioassay of biological samples such as feces and urine. In order to establish a method for internal exposure dose estimation for urine samples, we evaluated sample pretreatment by wet digestion prior to radionuclide measurement by inductively coupled plasma mass spectrometry. Wet digestion method was selected so that not only thorium and uranium but also ⁹⁰Sr and ⁹⁹Tc in urine samples could be determined. Analysis of thorium and uranium in tap water of Hirosaki city, and of mineral water and urine samples provided by the residents of Fukushima prefecture were performed using the resulting bioassay protocol. The concentrations of ²³²Th in urine samples were 4.8 to 43.1 ppt and ²³⁸U concentrations were 25.8 to 133.2 ppt, which were higher than those in the corresponding water and mineral water samples. These values are thought to be affected by the thorium and uranium content of both food and drinking water. Pretreatment of urine samples for thorium and uranium for inductively coupled plasma mass spectrometry was established as a bioassay for internal exposure dose estimation.