A personal noise dosimeter was newly developed using a commercially sold portable digital audio tape recorder (DAT) and an electret condenser microphone. The method allows one to measure noise fluctuations during working hours at octave or one-third-octave band levels. From these exposure data, the critical band level for a noise-induced temporary threshold shift (NITTS) was estimated. The fluctuation of an NITTS induced by that noise exposure during working hours could thus be estimated by means of the formulas of NITTS growth and the unit step function method. This dosimeter is expected to contribute to more precise hearing risk assessment using estimated NITTS values. Especially, the maximum NITTS values that appeared during noise exposure could be useful to quantitatively evaluate the effect on hearing of noise controls, such as noise reduction at sound sources, shortening of exposure time by taking rests, or wearing ear protectors. Therefore, this technique will be widely applicable in different workplaces and could contribute to promoting more effective noise control of various types.