Feast et al. (1989) obtained observationally a period-luminosity (P-L) relation of Mira variables in LMC. Basically, fundamental periods of stellar pulsation should be determined by two quantities —a mass and a radius of the star. The existence of a P-L relation implies a special condition that reduces a function of two variables into a function of only a single variable. Though the AGB appears as a thin line in the HR diagram like the Cepheid instability strip, it is an “asymptotically” merged line of evolutionary tracks of stars in a range of about 0.5-3 solar masses. This fact means that the masses of stars on a point of the AGB are not unique though the radii and the effective temperatures are unique, respectively. Therefore we cannot expect, in the case of AGB stars, that a period is reduced to a function of a single variable.