Seven isolates of three Sordaria species which were antagonistic to Armillaria mellea were selected as biocontrol agents, and tested against soilborne diseases, caused by Pythium aphanidermatum, and/or Dematophora necatrix, of cucumber, spinach, komatsuna, and Japanese black pine seedlings using the soil-over-agar-culture-inoculation methods under the growth chamber, and semi-field conditions. The effects of the antagonists were evaluated based on the emergence, and healthy seedling rates in the soil over the co-cultures of pathogen and antagonist in various combinations in plates, or in the artificially infested soils, directly applied by pouring potato dextrose broth cultures of antagonists or sowing the seeds treated with antagonists. Biocontrols of the soilborne plant diseases by Sordaria spp. were almost successful in any treatment in this study.