Abstract: A wastewater treatment device includes a first biological treatment device (10) that treats wastewater through a membrane-separated activated sludge method; a second biological treatment device (20) that treats wastewater through a biofilm method; measurement units (31, 32) that measure load concentration and flow rate in the wastewater flowing upstream of the first biological treatment device (10) and the second biological treatment device (20); and a control unit (30) that calculates a load quantity from the load concentration and the flow rate, determines, according to the load quantity, a distribution ratio between a flow rate of the wastewater supplied to the first biological treatment device (10) and a flow rate of the wastewater supplied to the second biological treatment device (20), and adjusts the flow rate of the wastewater supplied to the first biological treatment device (10) and the second biological treatment device (20), based on the distribution ratio.