PURPOSE: To prevent oil leakage from a space between a front part case of mission case and am intermediate case by a load working on a car body through works at the time of soil loading work or excavation work by fitting a front loader and a backhoe in a tractor, and to provide a seal structure that bears well fatigue due to compression.

CONSTITUTION: An interposing plate 20 made of metal is sandwiched in a connected surface between a front part case 9 of transmission case and an intermediate case 10, and the interposing plate 20 is divided into inner and outer directions of the transmission case, while an elastic seal material 22 connecting to the front part case 9 as well as the intermediate case 10 is provided between the inner and outer interposing plates 20a and 20b, and sealing of the connected surface between the front part case 9 and the intermediate case 10 is thus achieved.