PURPOSE: To stabilize the handling of an experimental instrument and make easy the cleaning thereof by providing a recessed part on some part of top plate and also an exhaust orifice on the vertical wall of the recessed part.

CONSTITUTION: A recessed part 5 is formed on the top plate of an experimental table 4, and an exhaust orifice 2 on the vertical wall of the recessed part 5. An experiment takes place on the top plate or the recessed part 5, and a heavier gas than air is sucked from the exhaust orifice 2 of the recessed part 5 to be exhausted from an exhaust duct 3 through an exhaust passage 6. This exhaust passage 6 can be formed using an exhaust wall 7 and an exhaust orifice 3 can be provided. In addition, wire netting can be used as a partition and a direction-variable fin 2' can be installed. In this case, it is recommended that one side of the recessed part 5 consists of a square notched part measuring about 40 to 60cm.