PROBLEM TO BE SOLVED: To obtain a new low temperature sensitive lactobacillus strain capable of easily preparing a fermented milk of which acid value elevation during its preservation is suppressed, by using it as a lactobacillus starter, since it has different acid forming capabilities at each of prescribed temperature ranges. SOLUTION: This low temperature sensitive lactobacillus strain has an excellent acid forming capability in 37-43 deg.C temperature range, and also inferior acid forming capability at 15-25 deg.C temperature range. Further, the low temperature sensitive lactobacillus strain belonging to the genus Streptococcus, is Streptococcus thermophilus SBT0144 (FERM P-16638), and it is preferable to produce a fermented milk of which acid value elevation is suppressed by including the low temperature sensitive lactobacillus strain. Further it is useful for producing a high quality yogurt.