A sludge concentrating apparatus comprising a sludge tank for storing sludge and a filter cloth having a shape of endless belt which turns between upper and lower rollers, further comprising filtering means of which at least the lowermost of rollers is immersed in the sludge, washing means of the filter cloth, and a filtrate chamber having a filtrate introducing port in contact with the filter cloth and a weir, wherein a sludge distributing tank serving as sludge introducing means into the sludge tank is provided near the upper end of the filtrate introducing port, and a concetrated sludge discharge tank serving as concertrated sludge introducing means with the filter cloth in between; said sludge concentrating apparatus further comprising coagulant adding means for adding an acid, and a flow adjustment device serving as a sludge uniform feeding means which feeds sludge fed into the sludge tank uniformly in the width direction of the filter cloth. It is thus possible to efficiently concentrate sludge produced in a sewage disposal plant in a large quantity without depending upon properties of sludge.