A construction machine comprising: an automotive lower traveling structure; an upper revolving structure rotatably mounted on said lower traveling structure; and a working mechanism tiltably provided on said upper revolving structure, wherein said upper revolving structure includes: a revolving frame having a bottom plate and left and right vertical plates (5B, 5C) provided on said bottom plate and having a connecting pin located on a front side of each of said vertical plates (5B, 5C) for mounting said working mechanism, an engine mounted on a rear side of said revolving frame, and an oil reservoir tank having a front end located on a rear side from said connecting pin of said revolving frame and arranged on an outer side of one vertical plate of said vertical plates (5B, 5C), a housing case is provided in said revolving frame at a position on the front side of said oil reservoir tank, characterized in that: said housing case is constituted by including a case-fixing bracket mounted on said revolving frame, a lower accommodating case arranged by surrounding said case-fixing bracket and in which a reducing agent tank storing a reducing agent for purifying an exhaust gas from said engine is accommodated, and an upper accommodating case located on an upper side of said lower accommodating case, said case-fixing bracket is formed by a left leg part, a right leg part, a mounting platform provided on each of said leg parts (18, 19) across said reducing agent tank, and said mounting platform of said case-fixing bracket connects a top surface of said lower accommodating case and a lower surface of said upper accommodating case. an automotive lower traveling structure; an upper revolving structure rotatably mounted on said lower traveling structure; and a working mechanism tiltably provided on said upper revolving structure, wherein said upper revolving structure includes: a revolving frame having a bottom plate and left and right vertical plates (5B, 5C) provided on said bottom plate and having a connecting pin located on a front side of each of said vertical plates (5B, 5C) for mounting said working mechanism, an engine mounted on a rear side of said revolving frame, and an oil reservoir tank having a front end located on a rear side from said connecting pin of said revolving frame and arranged on an outer side of one vertical plate of said vertical plates (5B, 5C), a housing case is provided in said revolving frame at a position on the front side of said oil reservoir tank, characterized in that: said housing case is constituted by including a case-fixing bracket mounted on said revolving frame, a lower accommodating case arranged by surrounding said case-fixing bracket and in which a reducing agent tank storing a reducing agent for purifying an exhaust gas from said engine is accommodated, and an upper accommodating case located on an upper side of said lower accommodating case, said case-fixing bracket is formed by a left leg part, a right leg part, a mounting platform provided on each of said leg parts (18, 19) across said reducing agent tank, and said mounting platform of said case-fixing bracket connects a top surface of said lower accommodating case and a lower surface of said upper accommodating case. a revolving frame having a bottom plate and left and right vertical plates (5B, 5C) provided on said bottom plate and having a connecting pin located on a front side of each of said vertical plates (5B, 5C) for mounting said working mechanism, an engine mounted on a rear side of said revolving frame, and an oil reservoir tank having a front end located on a rear side from said connecting pin of said revolving frame and arranged on an outer side of one vertical plate of said vertical plates (5B, 5C), a housing case is provided in said revolving frame at a position on the front side of said oil reservoir tank, characterized in that: said housing case is constituted by including a case-fixing bracket mounted on said revolving frame, a lower accommodating case arranged by surrounding said case-fixing bracket and in which a reducing agent tank storing a reducing agent for purifying an exhaust gas from said engine is accommodated, and an upper accommodating case located on an upper side of said lower accommodating case, said case-fixing bracket is formed by a left leg part, a right leg part, a mounting platform provided on each of said leg parts (18, 19) across said reducing agent tank, and said mounting platform of said case-fixing bracket connects a top surface of said lower accommodating case and a lower surface of said upper accommodating case. said housing case is constituted by including a case-fixing bracket mounted on said revolving frame, a lower accommodating case arranged by surrounding said case-fixing bracket and in which a reducing agent tank storing a reducing agent for purifying an exhaust gas from said engine is accommodated, and an upper accommodating case located on an upper side of said lower accommodating case, said case-fixing bracket is formed by a left leg part, a right leg part, a mounting platform provided on each of said leg parts (18, 19) across said reducing agent tank, and said mounting platform of said case-fixing bracket connects a top surface of said lower accommodating case and a lower surface of said upper accommodating case.