To provide a polyester binder fiber used for obtaining a wet nonwoven fabric for a filter reduced in its shrinkage at time of drying or heat treatment in a papermaking process and excellent in nonwoven fabric properties (strength, or the like).

The polyester binder fiber for use in the wet nonwoven fabric for the filter has a fiber length of 3-25 mm and a sheet area shrinkage factor defined as follows of 30% or less. Herein, the sheet area shrinkage factor (%) is a value calculated according to {(A0-A1)/A0}×100. A0 in the formula is the area of a sheet before shrinkage treatment and A1 is the area of the sheet after shrinkage treatment. Herein, the sheet has a weight basis of about 50g/m2 and is formed from an aqueous dispersion containing 80 wt.% of the polyester binder fiber and 20 wt.% of NBKP using a square sheet machine and subjected to slackening and shrinkage treatment at 120°C for 5 min by a hot air circulating type dryer after dried. Further, the wet nonwoven fabric for the filter contains 5-50 wt.% of the polyester binder fiber.