PURPOSE: To prevent sagging of filter cloths and to perform stable dehydration by making the tensing force of the filter cloth on the inside of the final pressing roll out of two sheets of the filter cloths of a traveling filter cloth type dehydrator larger than the tensing force on the outside filter cloth.

CONSTITUTION: The hydrous sludge flocculated in an agitator 30 is supplied to a gravity dehydrating section of filter cloths 1, 2 via a chute 31 and is grasped between the filter cloths 1 and 2 in a pressing roll 6. The sludge is then conveyed to succeeding pressing rolls 7W14 and is successively dehydrated under pressure. The dehydrated cake is scraped off by scrapers 32, 33 by opening the superposed cloths 1, 2 and is carried to the outside of the system. The tensing force on the cloth 2 which comes to the inside of the final pressing roll 14 out of two sheets of the filter cloths 1, 2 is made larger than the tensing force on the cloth 1 by filter cloth tensing devices 28, 29. If there is no hydrous sludge between two sheets of the superposed filter cloths, the tensing force on two sheets of the filter cloths is so controlled as to be made the same by a sludge detector.