A urea water tank for a construction machine comprising: a tank body for reserving urea water that is supplied to an exhaust gas post-treatment device for purifying an exhaust gas of an engine mounted on the construction machine body (2, 3); and a sensor unit that is disposed to be inserted in said tank body and includes a sensor to detect a state of the urea water, characterized in that: the urea water tank for the construction machine includes: an opening that is disposed on a top surface part of said tank body and opens more largely than an external dimension of said sensor unit; a closing member that is disposed on the said top surface part of said tank body to close said opening; and a filter which is formed as a tubular body to surround said sensor unit, and inserted in said opening of said tank body from a tip end side of said filter, and having a base end (24A3) mounted on said top surface part of said tank body by using said closing member, wherein said opening disposed in said top surface part of said tank body is a unit insertion opening for inserting said sensor unit, said sensor unit is mounted to said closing member, said filter includes a flange part extending in a radial outside of said filter on said base end (24A3), said filter is inserted in said tank body through said unit insertion opening from a tip end-side, said flange part of said filter is mounted to be tightly held between said closing member and said top surface part of said tank body by said closing member for closing said unit insertion opening, and said bottom surface part of said tank body is provided with a drain plug that is positioned inside of said filter to discharge the urea water. a tank body for reserving urea water that is supplied to an exhaust gas post-treatment device for purifying an exhaust gas of an engine mounted on the construction machine body (2, 3); and a sensor unit that is disposed to be inserted in said tank body and includes a sensor to detect a state of the urea water, characterized in that: the urea water tank for the construction machine includes: an opening that is disposed on a top surface part of said tank body and opens more largely than an external dimension of said sensor unit; a closing member that is disposed on the said top surface part of said tank body to close said opening; and a filter which is formed as a tubular body to surround said sensor unit, and inserted in said opening of said tank body from a tip end side of said filter, and having a base end (24A3) mounted on said top surface part of said tank body by using said closing member, wherein said opening disposed in said top surface part of said tank body is a unit insertion opening for inserting said sensor unit, said sensor unit is mounted to said closing member, said filter includes a flange part extending in a radial outside of said filter on said base end (24A3), said filter is inserted in said tank body through said unit insertion opening from a tip end-side, said flange part of said filter is mounted to be tightly held between said closing member and said top surface part of said tank body by said closing member for closing said unit insertion opening, and said bottom surface part of said tank body is provided with a drain plug that is positioned inside of said filter to discharge the urea water. an opening that is disposed on a top surface part of said tank body and opens more largely than an external dimension of said sensor unit; a closing member that is disposed on the said top surface part of said tank body to close said opening; and a filter which is formed as a tubular body to surround said sensor unit, and inserted in said opening of said tank body from a tip end side of said filter, and having a base end (24A3) mounted on said top surface part of said tank body by using said closing member, wherein said opening disposed in said top surface part of said tank body is a unit insertion opening for inserting said sensor unit, said sensor unit is mounted to said closing member, said filter includes a flange part extending in a radial outside of said filter on said base end (24A3), said filter is inserted in said tank body through said unit insertion opening from a tip end-side, said flange part of said filter is mounted to be tightly held between said closing member and said top surface part of said tank body by said closing member for closing said unit insertion opening, and said bottom surface part of said tank body is provided with a drain plug that is positioned inside of said filter to discharge the urea water.