To reveal the detail of the internal structure, the relationship between chromospheric activity and the Rossby number, (= rotational period / convective turnover time ), has been extensively examined for main-sequence stars. The goal of our work is to apply the same methods to pre-main-sequence (PMS) stars and identify the appropriate model of for them. Yamashita et al. (2020) investigated the relationship between and strengths of the Ca I\hspace{-.1em}I infrared triplet ( ) emission lines of 60 PMS stars. Their equivalent widths are converted into the emission line to the stellar bolometric luminosity ratio (). The 54 PMS stars have and show as large as the maximum of the zero-age main-sequence (ZAMS) stars. However, because all was saturated against , it was not possible to estimate the appropriate model for the PMS stars. We noticed that Mg I emission lines at is an optically thin chromospheric line, appropriate for determination of the adequate for PMS stars. Using the archive data of the AAT/the University College London Echelle Spectrograph, we investigated the Mg I line of 52 ZAMS stars. After subtracting photospheric absorption component, the Mg I line is detected as an emission line in 45 ZAMS stars, whose is between and . The Mg I line is not saturated yet in "the saturated regime for the Ca I\hspace{-.1em}I emission lines", i.e. . Therefore, the adequate for PMS stars can be determined by measuring of their values.