A multi-purpose water tank in which an opening and closing cover is installed to selectively open and close an opening formed in a top of the water tank and an inside thereof is filled with water, wherein: a discharging and filtering means configured to discharge water accommodated in the inside of the water tank is installed in a lower portion of a front surface of the water tank; a bottom surface of the inside of the water tank is formed as an inclined surface inclined downward from edges thereof to a center thereof so that a small amount of water flows to the center along with impurities remaining on the bottom surface of the inside of the water tank when the small amount of water remains in the water tank; an accommodation and filtering depression configured to accommodate water and impurities collected at the center along the inclined surface is formed to be inclined downward from the center of the water tank to a direction of the discharging and filtering means; the discharging and filtering means is configured to include: a hollow coupler installed in a lower side surface of an outside of the water tank to communicate with the accommodation and filtering depression; a filtering rod installed to selectively enter and exit in a direction of the accommodation and filtering depression through the coupler, and installed such that one side end thereof in a lengthwise direction thereof is caught on an inside of the coupler and a remaining side end thereof in the lengthwise direction is supported on a bottom surface of the accommodation and filtering depression so that a portion of the filtering rod between the one side end and the remaining side end in the lengthwise direction is spaced from the bottom surface of the accommodation and filtering depression by a predetermined distance; and a connection coupler detachably installed on the coupler, and connected to a water flow conduit part in a state of accommodating the coupler; and the water flow conduit part includes a connection pipe connected to the connection coupler, and a branch part formed to be branched off from the connection pipe, and configured to include a water introduction conduit for backwashing the filtering rod by introducing water into an inside of the filtering rod and a water discharge conduit for discharging water accommodated in the water tank; and cutoff valves are installed on the water introduction conduit and the water discharge conduit, respectively. a discharging and filtering means configured to discharge water accommodated in the inside of the water tank is installed in a lower portion of a front surface of the water tank; a bottom surface of the inside of the water tank is formed as an inclined surface inclined downward from edges thereof to a center thereof so that a small amount of water flows to the center along with impurities remaining on the bottom surface of the inside of the water tank when the small amount of water remains in the water tank; an accommodation and filtering depression configured to accommodate water and impurities collected at the center along the inclined surface is formed to be inclined downward from the center of the water tank to a direction of the discharging and filtering means; the discharging and filtering means is configured to include: a hollow coupler installed in a lower side surface of an outside of the water tank to communicate with the accommodation and filtering depression; a filtering rod installed to selectively enter and exit in a direction of the accommodation and filtering depression through the coupler, and installed such that one side end thereof in a lengthwise direction thereof is caught on an inside of the coupler and a remaining side end thereof in the lengthwise direction is supported on a bottom surface of the accommodation and filtering depression so that a portion of the filtering rod between the one side end and the remaining side end in the lengthwise direction is spaced from the bottom surface of the accommodation and filtering depression by a predetermined distance; and a connection coupler detachably installed on the coupler, and connected to a water flow conduit part in a state of accommodating the coupler; and the water flow conduit part includes a connection pipe connected to the connection coupler, and a branch part formed to be branched off from the connection pipe, and configured to include a water introduction conduit for backwashing the filtering rod by introducing water into an inside of the filtering rod and a water discharge conduit for discharging water accommodated in the water tank; and cutoff valves are installed on the water introduction conduit and the water discharge conduit, respectively.