The filtration device mainly contains a hollow body member having a water inlet and a water outlet at two ends; a number of open-top container members vertically stacked and reliably positioned, inside the body member; and a number of agent members placed inside the container members respectively. Each of the container members has a number of through holes arranged spirally at the bottom so that the water is forced into a swirl as it flows through the container members to make full utilization of the agent members. The body member is sealed by a cover that can be easily twisted close or open. As such, the standardized container members can be easily accessed and the agent members can be easily replaced.