Isomerizations of methyldibenzothiophene from Neogene mudstones in northeast Japan were examined as a maturity parameter of organic matter during the diagenesis to catagenesis stage．The relationship between methyldibenzothiophene（MDBT）ratio（MDR：4-MDBT/1-MDBT）and vitrinite reflectance（Ro）from the Tojo TS-1 well，which underwent subsidence at low heating rate，can be described by the equation：MDR=15.1 Ro-5.35（n=11，r=0.85，Ro=0.4-0.7%）．This equation is similar to that from Elmworth 6-28-68-13 W 6 M in Western Canada．New combination of the 1-MDBT，（2- ＋ 3-）MDBT and 4-MDBT isoomers are used to be a new maturity parameter which is useful in the high maturity stage，up to 3% Ro．The ratio（2- ＋ 3-）MDBT to（1- + 2- + 3- + 4-）MDBT increases from 0.2 to 0.6 over the range 0.4-3.0% Ro．The ratio（2- + 3-）MDBT to 4-MDBT was constant at about 0.5 in the range 0.4-1.5% Ro，and then increases from 0.5 to 4.5 in the range 1.5-2.9% Ro．Therefore，the（2- + 3-）/（1- + 2- + 3- + 4-）MDBT ratio is possibly on effective maturity indicator through the oil generation stage to higher maturity stages such as the dry gas zone．After the catagenesis stage，（2- ＋ 3-）/（4-）MDBT ratio and high Ro gradients could assist interpretation of scattered（2- ＋ 3-）/（1- + 2- + 3- + 4-）MDBT ratios with low gradient．