A liquid separation device is provided, and is capable of suppressing the lowering of filtration function due to an increase in flow channel resistance of permeated liquid which results in a separation membrane falling in a groove of a permeated liquid flow channel material along with accompanied breakage of the separation membrane surface. A permeated liquid flow channel material is disposed on the back side of a separation membrane composed of a sheet-like material having a linear groove and a linear crest alternately arrayed on one surface or both surfaces, wherein a groove width of the linear groove in the sheet-like material is 10 to 200 μm, and a ratio of the groove width of the linear groove to the pitch of the linear groove is 0.45 or more.