The Jeans scale is estimated during the coexistence epoch of quark-gluon and hadron phases in the first-order QeD phase transition. It is shown that, contrary to previous claims, reduction of the sound speed is so little that the phase transition does not affect evolution of cosmological density fluctuations appreciably. 173 As is well known, the Jeans length scale, above which density fluctuations can grow, is given by the sound speed Cs multiplied by the dynamical time scale. In the radiation-dominant stage of the early universe, Cs is equal to 3- 1 / 2 times the light speed, which we take to be unity here, so density fluctuations can grow only on super-horizon scales. Recently, however, Jedamzik 1) and, independently, Schmid, Schwarz and Widerin 2) proposed that if a cosmological quark-hadron phase transition is of first order, which is supported by some of the recent lattice simulations, 3) the sound speed effectively vanishes during the coexistence period of the quark-gluon and the hadron phases. If this were true, the Jeans scale would become vanishingly small, and even sub-horizon-scale fluctuations should grow during this epoch. This temporal decline of the Jeans length would imprint a nontrivial feature on the processed spectrum of fluctuations around the solar-mass scale. 2) Furthermore, it has been suggested that such anomalous growth of perturbations could induce efficient formation of primor dial black holes (PBH's).I) The resultant mass function of the PBH's would have a peak on the solar-mass scale which is about the scale of MACHO's. Thus the idea raised in Refs. 1) and 2) is very interesting and may be astrophysically important. Unfortunately, however, the actual evolution of the sound speed, as well as that of the Jeans length, has not been given in either paper, and their claims are limited to a qualitative level. In the present paper we present the result of an explicit calculation of the sound speed using the bag model. 4) We first review how the quark-hadron phase transi tion proceeds in this model and then calculate the sound speed under two distinct conditions.