N,N-Diethyl-m-methylbenzamide (DEET)-based repellents may provoke dermal toxicity in infants and children. For use against mosquitoes, various plant-derived essential oils are considered useful alternatives to such synthetic chemicals. The present study was undertaken to scientifically evaluate the repellency of 33 essential oils against female Culex pipiens pallens adults. By using the ASTM E951-94 apparatus with human volunteers, we showed that the 33 essential oils exhibited varying degrees of mosquito repellency. Four oils, namely, clove bud, clove leaf, juniperberry and majoram, showed good repellency at a concentration of 0.005 mg/$cm^2$. The clove bud and clove leaf oils had higher repellency than citronella known as a commercial repellent ingredient. Analysis of the clove bud and clove leaf oils by GC and GC-MS revealed their major components. Repellency analysis of these individual components indicated that isoeugenol, a major component of clove bud oil, had the highest repellency, while eugenol, a major component of both clove oils, was also more repellent than citronella. Clove bud oil, which was the most repellent of all tested oils, exhibited the prolonged repellency when mixed with vanillin for long-term use. Clove bud oil clearly had higher repellency than commercial repellents. Further research on means to prolong the repellency of this oil will be necessary before it can replace DEET.