A construction machine comprising: a support frame forming a support structural member for a traveling structure (2, 3); an engine mounted at the rear side of said support frame; a cab which is positioned at the front side of said support frame and comprises a floor surface (11, 31, 41, 51), side surface portions (19A to 19D) in the front and rear directions and in the right and left directions and a top surface portion, and in which an operator's seat for seating of an operator is arranged on said floor surface (11, 31, 41,51); a NOx purifying device provided in an exhaust pipe of said engine and equipped with a urea selective reduction catalyst for removing nitrogen oxides in an exhaust gas; a urea water tank (23, 33, 43, 53) formed of a hollow container for reserving urea water as a reducing agent and Including a water supply port for supplying the urea water; and a connecting pipe connecting said urea water tank (23, 33, 43, 53) and said exhaust pipe of said engine, wherein said urea water tank (23, 33,43, 53) is arranged in said cab which is in a preferred temperature environment for preventing freeze and crystallization of the urea water, characterized In that a tank accommodating recessed portion (13, 32, 42, 52) is provided on said floor surface (11, 31, 41, 51) of said cab by recessing a part of said floor surface (11, 31, 41, 51) downwards between said support frame and said floor surface (11, 31, 41, 51); said urea water tank (23, 33, 43, 53) is accommodated in said tank accommodating recessed portion (13, 32, 42, 52); and said water supply port is provided to be opened to one side surface portion of said side surface portions (19A to 19D) of said cab for supplying the urea water to said urea water tank (23, 33, 43, 53) from an outside of said cab. a support frame forming a support structural member for a traveling structure (2, 3); an engine mounted at the rear side of said support frame; a cab which is positioned at the front side of said support frame and comprises a floor surface (11, 31, 41, 51), side surface portions (19A to 19D) in the front and rear directions and in the right and left directions and a top surface portion, and in which an operator's seat for seating of an operator is arranged on said floor surface (11, 31, 41,51); a NOx purifying device provided in an exhaust pipe of said engine and equipped with a urea selective reduction catalyst for removing nitrogen oxides in an exhaust gas; a urea water tank (23, 33, 43, 53) formed of a hollow container for reserving urea water as a reducing agent and Including a water supply port for supplying the urea water; and a connecting pipe connecting said urea water tank (23, 33, 43, 53) and said exhaust pipe of said engine, wherein said urea water tank (23, 33,43, 53) is arranged in said cab which is in a preferred temperature environment for preventing freeze and crystallization of the urea water, characterized In that a tank accommodating recessed portion (13, 32, 42, 52) is provided on said floor surface (11, 31, 41, 51) of said cab by recessing a part of said floor surface (11, 31, 41, 51) downwards between said support frame and said floor surface (11, 31, 41, 51); said urea water tank (23, 33, 43, 53) is accommodated in said tank accommodating recessed portion (13, 32, 42, 52); and said water supply port is provided to be opened to one side surface portion of said side surface portions (19A to 19D) of said cab for supplying the urea water to said urea water tank (23, 33, 43, 53) from an outside of said cab.