A construction machine comprising: a vehicle body frame that is formed as a support structure; a counterweight that is provided in a rear end to said vehicle body frame for acting as a weight balance to a front device; an engine that is positioned in a front side to said counterweight and is mounted on said vehicle body frame in a horizontal state; a hydraulic pump that is provided to be positioned in one side of the longitudinal direction of said engine; an exhaust gas post-treatment device that is connected to an exhaust gas side of said engine to be positioned above said hydraulic pump and performs post-treatment of an exhaust gas discharged from said engine; a firewall that is provided as a partition between an engine room in which said engine and said exhaust gas post-treatment device are arranged and a pump room in which said hydraulic pump is arranged; a cooling fan that is provided in said engine room to be positioned on the other side of the longitudinal direction of said engine, and sucks in outside air as cooling air; a heat exchanger that is provided on said vehicle body frame to face said cooling fan and cools fluid with the cooling air sucked by said cooling fan; and an exterior cover that is positioned in front of said counterweight to cover said engine, said hydraulic pump, said exhaust gas post-treatment device and said heat exchanger, characterized in that; said firewall includes a front surface plate mounted to be positioned in front of said hydraulic pump, a rear surface plate that is positioned in rear of said hydraulic pump and is provided to face said counterweight, and a partition plate that is positioned between said hydraulic pump and said exhaust gas post treatment device and provided by being extended between said front surface plate and said rear surface plate, said rear surface plate is provided with an outflow opening for outflow of the cooling air sucked in said engine room toward said counterweight, in a height position equivalent to said exhaust gas post-treatment device, and said counterweight includes an exhaust opening in a position facing said outflow opening to discharge the cooling air flowing out from said outflow opening to an exterior, said firewall includes a space forming plate which has an upper surface portion lower than said outflow opening, wherein an airflow passage part is formed between said partition plate and said rear surface plate for causing the cooling air to flow from said engine room toward said outflow opening by said space forming plate. a vehicle body frame that is formed as a support structure; a counterweight that is provided in a rear end to said vehicle body frame for acting as a weight balance to a front device; an engine that is positioned in a front side to said counterweight and is mounted on said vehicle body frame in a horizontal state; a hydraulic pump that is provided to be positioned in one side of the longitudinal direction of said engine; an exhaust gas post-treatment device that is connected to an exhaust gas side of said engine to be positioned above said hydraulic pump and performs post-treatment of an exhaust gas discharged from said engine; a firewall that is provided as a partition between an engine room in which said engine and said exhaust gas post-treatment device are arranged and a pump room in which said hydraulic pump is arranged; a cooling fan that is provided in said engine room to be positioned on the other side of the longitudinal direction of said engine, and sucks in outside air as cooling air; a heat exchanger that is provided on said vehicle body frame to face said cooling fan and cools fluid with the cooling air sucked by said cooling fan; and an exterior cover that is positioned in front of said counterweight to cover said engine, said hydraulic pump, said exhaust gas post-treatment device and said heat exchanger, characterized in that; said firewall includes a front surface plate mounted to be positioned in front of said hydraulic pump, a rear surface plate that is positioned in rear of said hydraulic pump and is provided to face said counterweight, and a partition plate that is positioned between said hydraulic pump and said exhaust gas post treatment device and provided by being extended between said front surface plate and said rear surface plate, said rear surface plate is provided with an outflow opening for outflow of the cooling air sucked in said engine room toward said counterweight, in a height position equivalent to said exhaust gas post-treatment device, and said counterweight includes an exhaust opening in a position facing said outflow opening to discharge the cooling air flowing out from said outflow opening to an exterior, said firewall includes a space forming plate which has an upper surface portion lower than said outflow opening, wherein an airflow passage part is formed between said partition plate and said rear surface plate for causing the cooling air to flow from said engine room toward said outflow opening by said space forming plate.