To provide a fuel volume measuring device for measuring the fuel amount independently of a fuel meter.

A pressure introducing mechanism is provided which can introduce pressure into a fuel tank by means of a known pressure introducing characteristics. Further, there is provided an inner pressure sensor for detecting inner pressure of the tank. After initiating pressure introduction by the pressure introducing mechanism, a steady pressure reaching time TSAT is measured that is required for the inner pressure of the tank to reach steady pressure (step 100 to 104). By bringing the measured TSAT into check with a TSAT-VFUEL map for defining the relationship between TSAT and an estimated fuel volume VFUEL, the estimated fuel volume VFUEL is estimated (step 106).