This true density measurement device (10) is a device that is used to measure the true density of a sample (100) by a gas phase substitution method, and is provided with: a sample chamber (30) for accommodating the sample (100), the sample chamber (30) being pressurized through the introduction of an inert gas; and an expansion chamber (50) into which is released the inert gas filling the sample chamber (30). The expansion chamber (50), under conditions of normal use, is opened and closed by a detachable cover (42), and the volume thereof is modified by insertion or withdrawal of a volume modification member (55).