A construction machine comprising: a vehicle body frame (5, 5') forming a support structural body of a vehicle body (2, 3), an engine mounted on said vehicle body frame (5, 5'), an exhaust gas post-treatment device connected to an exhaust side of said engine and performing post-treatment of an exhaust gas exhausted from said engine, a reduction agent tank storing a reduction agent to be supplied to said exhaust gas post-treatment device, a tank bracket (23, 42, 52) holding said reduction agent tank on said vehicle body frame (5, 5'); wherein said tank bracket (23, 42, 52) is formed by a square-shaped bottom surface part having a first frame body, a second frame body, a third frame body, and a fourth frame body, a first side surface part (25, 53) supported by said first frame body of said bottom surface part and provided in a vertical direction, and a second side surface part (26, 54) supported by said second frame body adjacent to said first frame body in each of said frame bodies (24A, 24B, 24C, 24D) of said bottom surface part, provided in the vertical direction, and forming an L-shaped wall surface in collaboration with said first side surface part (25, 53); and wherein said tank bracket (23, 42, 52) includes a bracket fixing portion located on said bottom surface part and fixing said tank bracket (23, 42, 52) to said vehicle body frame (5, 5'); characterized in that a fixing band connecting one side surface part in said first side surface part (25, 53) and said second side surface part (26, 54) to one frame body in said first to fourth frame bodies (24A to 24D) forming said bottom surface part facing said one side surface part sandwiching said reduction agent tank and fixing said reduction agent tank between said bottom surface part and said one side surface part is provided in said tank bracket (23, 42, 52); and said tank bracket (23, 42, 52) is fixed to said vehicle body frame (5, 5') by said bracket fixing portion in a state in which said reduction agent tank is sandwiched by said one side surface part and a part of said vehicle body frame (5, 5') or a member mounted on said vehicle body frame (5, 5'). a vehicle body frame (5, 5') forming a support structural body of a vehicle body (2, 3), an engine mounted on said vehicle body frame (5, 5'), an exhaust gas post-treatment device connected to an exhaust side of said engine and performing post-treatment of an exhaust gas exhausted from said engine, a reduction agent tank storing a reduction agent to be supplied to said exhaust gas post-treatment device, a tank bracket (23, 42, 52) holding said reduction agent tank on said vehicle body frame (5, 5'); wherein said tank bracket (23, 42, 52) is formed by a square-shaped bottom surface part having a first frame body, a second frame body, a third frame body, and a fourth frame body, a first side surface part (25, 53) supported by said first frame body of said bottom surface part and provided in a vertical direction, and a second side surface part (26, 54) supported by said second frame body adjacent to said first frame body in each of said frame bodies (24A, 24B, 24C, 24D) of said bottom surface part, provided in the vertical direction, and forming an L-shaped wall surface in collaboration with said first side surface part (25, 53); and wherein said tank bracket (23, 42, 52) includes a bracket fixing portion located on said bottom surface part and fixing said tank bracket (23, 42, 52) to said vehicle body frame (5, 5'); characterized in that a fixing band connecting one side surface part in said first side surface part (25, 53) and said second side surface part (26, 54) to one frame body in said first to fourth frame bodies (24A to 24D) forming said bottom surface part facing said one side surface part sandwiching said reduction agent tank and fixing said reduction agent tank between said bottom surface part and said one side surface part is provided in said tank bracket (23, 42, 52); and said tank bracket (23, 42, 52) is fixed to said vehicle body frame (5, 5') by said bracket fixing portion in a state in which said reduction agent tank is sandwiched by said one side surface part and a part of said vehicle body frame (5, 5') or a member mounted on said vehicle body frame (5, 5').