Provided is an apparatus for detecting property of fuel in a construction machine, with use of a sub-tank, the apparatus being capable of minimizing an influence on the detection given by water contained in fuel. The apparatus includes a sub-tank (4) between an engine (2) and a fuel tank (1) of the construction machine, a fuel introducing pipe (9) through which fuel is introduced to the sub-tank (4) from the fuel tank (1), a fuel discharger through which fuel in the sub-tank (4) is discharged to the engine (2), and a sensor (5). The sub-tank (4) has a bottom section having a bottom fuel outlet (7) for discharging fuel from the bottom section. The fuel discharger includes a bottom fuel discharging pipe (10) connected to the bottom fuel outlet (7).