PURPOSE: To eliminate the need for the idle operation before measurement, by feeding pure water to a wiping-off member made of a flexible material excellent in water absorption properties, such as Moltopren, before measurement in order to wet said member.

CONSTITUTION: Pure water is fed to a sponge 4 through a feed pipe 8 and a hollow supporting rod 5. Reaction tubes 2 containing residual fluid are inverted so as to be in an upside down condition, in order to discharge the residual fluid, and washed by means of both the ciry water jetted out from branch pipes 3a and 3b and the pure wate jetted out from a branch pipe 3c. When said washed reaction tube 2 stops at above the sponge 4, a motor 9 is actuated to move a moving member 6 upward so that the sponge 4 is inserted into the reaction tube 2. At the top dead point, the water contained in the sponge 4 is squeezed to become waterdrops, which are drained through a drain pipe 14. Then, drawing out the sponge 4 permits the waterdrops attached to the wall of the reaction tube 2 to be wiped off. By such a constitution, the need for the idle operation before measurement can be completely eliminated.