In the neutron irradiation field in boron neutron capture therapy, neutron rays and γ-rays are mixed, and it is not easy to selectively measure only neutron rays. The currently used gold activation method is expensive and not suitable for measuring the dose distribution with high spatial resolution. Therefore, there is increasing demand for a simple neutron measurement method for periodic inspections and treatment planning in boron neutron capture therapy (BNCT). Cd has a large nuclear reaction cross section solely for thermal neutrons, and converts thermal neutrons to γ-rays by the (n, γ) reaction. Therefore, in this study, we investigated a method for calculating the thermal neutron fluence by installing Cd as a neutron converter on Cr-doped Al2O3 (Al2O3:Cr), which is a thermoluminescence dosimeter, and measuring the converted γ-ray dose with the Al2O3:Cr. After a 30 min irradiation experiment using the Kyoto University research reactor, the amount of thermoluminescence (TL) increased 101-fold after installing a Cd converter compared with the case of no Cd converter. In addition, the dependence of the TL response on the irradiation time showed excellent proportionality, suggesting the possibility of selectively measuring only the thermal neutron fluence.