A true density measurement device for measuring a true density of an object to be measured with a gas phase substitution method, the true density measurement device comprising: a sample chamber that houses the object to be measured, and is pressurized by introducing an inert gas into the sample chamber ; an expansion chamber into which the inert gas having been introduced into the sample chamber is released, wherein the expansion chamber is opened and closed by a lid which can be attached or detached under normal usage conditions, and has a variable capacity that is changed by inserting or removing a capacity changing member ; a pressure detector for detecting pressures of the sample chamber and the expansion chamber ; a control unit configured to calculate the true density of an object to be measured based on the weight of the object and the detected pressures; characterized in that the control unit comprises: a notifying section that is configured to recommend use of the capacity changing member based on a relationship between a previously obtained capacity of the expansion chamber and a measurement error, wherein the notifying section is configured to calculate (S11) a capacity optimum for the expansion chamber based on a measurement result (S10) of the true density of the object obtained without using a capacity changing member , determine (S12) whether or not the expansion chamber capacity used in the measurement (S10) is an appropriate condition for reducing a measurement error to a minimum, and output (S13) an indication recommending an operator of the true density measurement device to use the capacity changing member , if the used expansion chamber capacity does not match the calculated optimum. a sample chamber that houses the object to be measured, and is pressurized by introducing an inert gas into the sample chamber ; an expansion chamber into which the inert gas having been introduced into the sample chamber is released, wherein the expansion chamber is opened and closed by a lid which can be attached or detached under normal usage conditions, and has a variable capacity that is changed by inserting or removing a capacity changing member ; a pressure detector for detecting pressures of the sample chamber and the expansion chamber ; a control unit configured to calculate the true density of an object to be measured based on the weight of the object and the detected pressures; a notifying section that is configured to recommend use of the capacity changing member based on a relationship between a previously obtained capacity of the expansion chamber and a measurement error, wherein the notifying section is configured to calculate (S11) a capacity optimum for the expansion chamber based on a measurement result (S10) of the true density of the object obtained without using a capacity changing member , determine (S12) whether or not the expansion chamber capacity used in the measurement (S10) is an appropriate condition for reducing a measurement error to a minimum, and output (S13) an indication recommending an operator of the true density measurement device to use the capacity changing member , if the used expansion chamber capacity does not match the calculated optimum. <EOS>