A construction machine comprising: an automotive lower traveling structure, an upper revolving structure provided rotatably on said lower traveling structure, and a working mechanism provided on a front side of said upper revolving structure and driven by a hydraulic actuator (5D, 5E, 5F), said upper revolving structure including: a revolving frame forming a support structural body; an engine provided on said revolving frame at a location on a rear part side of said revolving frame; a hydraulic pump driven by said engine and supplying a pressurized oil to said hydraulic actuator (5D, 5E, 5F); an assist power generator/electric motor coupled to said engine and said hydraulic pump and performing a power generator action and an electric motor action; a cooling fan for sucking an outside air as cooling air and supplying said cooling air toward said engine; a heat exchanger provided on said revolving frame at a location closer to an upstream side in a flow direction of said cooling air than said cooling fan and cooling a fluid by said cooling air; and an electric power storage device disposed on said revolving frame at a location closer to the upstream side in the flow direction of said cooling air than said heat exchanger and charging generated power by said assist power generator/electric motor or discharging the charged power, characterized in that; said electric power storage device has a box-shaped casing forming an outer wall and a battery module provided inside said casing and accommodating an electrolyte; and an electrolyte discharge pipe (30, 42, 52) for discharging an electrolyte mist and/or electrolyte injected out of said battery module at abnormality of said battery module to an outside of said upper revolving structure is provided on said casing. an automotive lower traveling structure, an upper revolving structure provided rotatably on said lower traveling structure, and a working mechanism provided on a front side of said upper revolving structure and driven by a hydraulic actuator (5D, 5E, 5F), said upper revolving structure including: a revolving frame forming a support structural body; an engine provided on said revolving frame at a location on a rear part side of said revolving frame; a hydraulic pump driven by said engine and supplying a pressurized oil to said hydraulic actuator (5D, 5E, 5F); an assist power generator/electric motor coupled to said engine and said hydraulic pump and performing a power generator action and an electric motor action; a cooling fan for sucking an outside air as cooling air and supplying said cooling air toward said engine; a heat exchanger provided on said revolving frame at a location closer to an upstream side in a flow direction of said cooling air than said cooling fan and cooling a fluid by said cooling air; and an electric power storage device disposed on said revolving frame at a location closer to the upstream side in the flow direction of said cooling air than said heat exchanger and charging generated power by said assist power generator/electric motor or discharging the charged power, characterized in that; said electric power storage device has a box-shaped casing forming an outer wall and a battery module provided inside said casing and accommodating an electrolyte; and an electrolyte discharge pipe (30, 42, 52) for discharging an electrolyte mist and/or electrolyte injected out of said battery module at abnormality of said battery module to an outside of said upper revolving structure is provided on said casing. a revolving frame forming a support structural body; an engine provided on said revolving frame at a location on a rear part side of said revolving frame; a hydraulic pump driven by said engine and supplying a pressurized oil to said hydraulic actuator (5D, 5E, 5F); an assist power generator/electric motor coupled to said engine and said hydraulic pump and performing a power generator action and an electric motor action; a cooling fan for sucking an outside air as cooling air and supplying said cooling air toward said engine; a heat exchanger provided on said revolving frame at a location closer to an upstream side in a flow direction of said cooling air than said cooling fan and cooling a fluid by said cooling air; and an electric power storage device disposed on said revolving frame at a location closer to the upstream side in the flow direction of said cooling air than said heat exchanger and charging generated power by said assist power generator/electric motor or discharging the charged power, characterized in that; said electric power storage device has a box-shaped casing forming an outer wall and a battery module provided inside said casing and accommodating an electrolyte; and an electrolyte discharge pipe (30, 42, 52) for discharging an electrolyte mist and/or electrolyte injected out of said battery module at abnormality of said battery module to an outside of said upper revolving structure is provided on said casing.