A small-sized construction machine comprising: a self-propelled lower traveling structure; an upper revolving structure that is rotatably mounted on said lower traveling structure; and a working mechanism that is provided on the front part side of said upper revolving structure, wherein said upper revolving structure includes: a revolving frame forming a support structural body, a counterweight provided on a rear side of said revolving frame, an engine located on the front side of said counterweight and mounted on said revolving frame, a cab provided on said revolving frame so as to cover said engine from above and on one side in a left-right direction of which an openable/closable door (17C2) is provided, and a fuel tank that has a fuel filling port and a cap that closes said fuel filling port and stores fuel to be supplied to said engine, wherein a fuel filling opening that is located on the side behind said door (17C2) and surrounds said fuel filling port of said fuel tank and a lid member that caps said fuel filling opening to be openable/closable are provided on an outer surface of one side in the left-right direction on which said door (17C2) of said cab is provided, characterized in that: a cap holder (21, 31, 51) that is attached to the inner surface side of said lid member in a state of facing an inner surface of said lid member with an interval in order to hold said cap removed from said fuel tank and a lock (22, 32) that locks said lid member from the outside in a state where said lid member closes said fuel filling opening are provided on said lid member, said lock (22, 32) has a lock lever (22C, 32A) that is composed of a long member, a base end of which is rotatably attached to said lid member and a leading end of which is allowed to enter the gap between said lid member and said cap holder (21, 31, 51), and that rotationally moves between a locking position where said lid member maintains a state of closing said fuel filling opening and an unlocking position where said lid member allows opening of said fuel filling opening. a self-propelled lower traveling structure; an upper revolving structure that is rotatably mounted on said lower traveling structure; and a working mechanism that is provided on the front part side of said upper revolving structure, wherein said upper revolving structure includes: a revolving frame forming a support structural body, a counterweight provided on a rear side of said revolving frame, an engine located on the front side of said counterweight and mounted on said revolving frame, a cab provided on said revolving frame so as to cover said engine from above and on one side in a left-right direction of which an openable/closable door (17C2) is provided, and a fuel tank that has a fuel filling port and a cap that closes said fuel filling port and stores fuel to be supplied to said engine, wherein a revolving frame forming a support structural body, a counterweight provided on a rear side of said revolving frame, an engine located on the front side of said counterweight and mounted on said revolving frame, a cab provided on said revolving frame so as to cover said engine from above and on one side in a left-right direction of which an openable/closable door (17C2) is provided, and a fuel tank that has a fuel filling port and a cap that closes said fuel filling port and stores fuel to be supplied to said engine, wherein a fuel filling opening that is located on the side behind said door (17C2) and surrounds said fuel filling port of said fuel tank and a lid member that caps said fuel filling opening to be openable/closable are provided on an outer surface of one side in the left-right direction on which said door (17C2) of said cab is provided, characterized in that: a cap holder (21, 31, 51) that is attached to the inner surface side of said lid member in a state of facing an inner surface of said lid member with an interval in order to hold said cap removed from said fuel tank and a lock (22, 32) that locks said lid member from the outside in a state where said lid member closes said fuel filling opening are provided on said lid member, said lock (22, 32) has a lock lever (22C, 32A) that is composed of a long member, a base end of which is rotatably attached to said lid member and a leading end of which is allowed to enter the gap between said lid member and said cap holder (21, 31, 51), and that rotationally moves between a locking position where said lid member maintains a state of closing said fuel filling opening and an unlocking position where said lid member allows opening of said fuel filling opening. a cap holder (21, 31, 51) that is attached to the inner surface side of said lid member in a state of facing an inner surface of said lid member with an interval in order to hold said cap removed from said fuel tank and a lock (22, 32) that locks said lid member from the outside in a state where said lid member closes said fuel filling opening are provided on said lid member, said lock (22, 32) has a lock lever (22C, 32A) that is composed of a long member, a base end of which is rotatably attached to said lid member and a leading end of which is allowed to enter the gap between said lid member and said cap holder (21, 31, 51), and that rotationally moves between a locking position where said lid member maintains a state of closing said fuel filling opening and an unlocking position where said lid member allows opening of said fuel filling opening.