A sludge concentrating device comprising a sludge tank storing sludge, a filtering means having an endless belt filter cloth revolving around upper and lower rollers with at least the lowermost roller immersed in the sludge, a filter cloth washing means, a filtrate chamber having a filtrate inlet close to the filter cloth and a weir, a sludge distributing tank serving as a means for introducing sludge into the sludge tank and disposed in the vicinity of the upper end of the filtrate inlet, and a concentrated sludge discharge tank serving as means for discharging concentrated sludge from the sludge tank and disposed in a position substantially opposed to the sludge introducing means with the filter cloth interposed therebetween, wherein the device further comprises a flocculant adding means for adding a flocculant to the sludge fed to the sludge tank, an acid adding means for adding an acid, and a rectifier serving as a uniform sludge feeding means for feeding sludge into the sludge tank uniformly widthwise of the filter cloth. Thus, the sludge produced in a sewage disposal plant can be concentrated in large quantities and efficiently without depending on its properties.