PURPOSE: To obtain a reactor of an efficient installation space, electric power consumption, etc. by segmenting the inside of the reactor by means of a partition plate having openings and using one thereof as a quick agitating section and the other as a slow agitating section.

CONSTITUTION: The inside of a reactor 1 is segmented to an upper half section and a lower half section by means of a partition plate 2 having openings, and a supply port 3 for raw liquid and a quick agitator 7 are provided in one of these to make a quick agitating section 8. A reactant outflow port 10 is provided to the other to make a slow agitating section 13. A flocculating agent, etc. are added to raw liquid such as sludge and these are quickly agitated, after which the agent is added and mixed to and with the sludge by slow agitation thereby allowing flocculation and reaction to take place effectively. The inside of the reactor is constituted efficiently in this way, and the installation space, electric power consumption, etc. are saved.