To provide a filter medium, a process for producing filter medium, a filtration device, a method for operating the filtration device, and a filtration system, which are capable of promptly regenerating the adsorption power by backwashing and realizing efficient operation of a filtration device. The filter medium of the present invention contains a carbon-based material in which a cumulative pore volume of pores having a pore radius of 2 nm or less is 25% or less with respect to a cumulative pore volume of pores having a pore radius of 50 nm or less.