The transient cooling of heat-generating materials contacted with a thermoelectric microcooler was discussed, and the operation of a fixed electric current was proposed as an alternative to the conventional operation of a variable electric current to obtain the maximum heat flux of cooling. The changes in temperature of the materials cooled by the thermoelectric cooler were obtained by numerical calculation of the unsteady one-dimensional energy equation. The proposed operation of a fixed current was found to be a useful alternative to the conventional operation. An approximate analysis was made on the basis of the results of the numerical calculation. The results showed that the transient response of cooling of material without heat generation could be expressed by the first-order delay. Comparisons with the numerical calculation indicated that the expression by the first-order delay is also applicable to the cooling of material with heat generation and that the time constant decreases with increasing rate of heat generation from the material.