A filter apparatus including: a casing including, in a sealed space provided therein, a fluid inflow chamber, a filtration chamber and a drain chamber; a filter element provided inside the filtration chamber to let a fluid pass therethrough from inside to outside to perform filtration; a backwash slide member movably provided inside the filter element and removing trapped matter adhering to an inner circumferential surface of the filter element by an inward flow generated by a difference in pressure inside and outside the filter element at the time of backwashing; a backwash pipe that lets the inward flow flow toward the drain chamber; an actuation cylinder that lets the fluid from the backwash pipe flow toward the drain chamber; and a fluid flowing unit allowing a fluid to flow, enabling a piston member to move inside the actuation cylinder.