The Tosa-no-Onagadori (briefly Onagadori) is one of the native Japanese chickens that is characterized by extremely long tail feathers in males, and thus designated as a “Special National Natural Treasure” of Japan. We investigated the timing of sexual maturity of the Onagadori and White Leghorn hens and the quality of the first 10 eggs laid by these young hens to obtain basic information for reproducing the Onagadori efficiently and also to assess their usefulness as research material for quantitative trait loci (QTL) analysis of egg-related traits. Onagadori and White Leghorn hens reached sexual maturity at around 237 and 192 days of age, respectively. The repeatability, a genetic parameter, of 22 egg quality traits including weight, size, and color of the eggshell, albumen, and yolk was estimated to be 0.14-0.61 and 0.15-0.83 for the Onagadori and White Leghorn, respectively, which indicated the stability of Onagadori eggs as well as the White Leghorn and further suggested that young Onagadori hens lay eggs with suitable quality to produce more offspring from their earliest egg laying stage. Principal component analysis with the 22 egg quality traits revealed five principal components explaining 79.70% of the total phenotypic variance. One-way ANOVA revealed differences between the two breeds in PC2 (heights of the albumen and yolk, size of the whole egg, and weights of the whole egg and albumen), PC3 (eggshell color), and PC4 (weight, thickness, and strength of the eggshell), which means that besides cultural assets the Onagadori is also valuable for QTL analysis of egg-related traits, for which phenotypic differences are quite useful for detecting corresponding loci.