A true density measurement device for measuring the true density of an object to be measured with a gas phase substitution method, the true density measurement device comprising: a manifold including a sample chamber for housing the object to be measured, and further including an expansion chamber wherein the sample chamber comprises an opening and a circumferential edge region around the opening; and a lid unit including a lid unit body comprising a first end part and a second end part, and a non-rotary lid that is used for closing the opening of the sample chamber and is pressed against the circumferential edge region around the opening to hermetically seal the sample chamber; characterized in that the true density measurement device further comprises: a floating joint with a rotation axis by means of which the lid is swingably attached between the first end part and the second end part of the lid unit body; and a device case covering the manifold and pivotally supporting the lid unit body at its second end part through a support unit comprising an axis of rotation, wherein the device case comprises a case opening; wherein the lid unit body functions as an arm that is used for an operation to open or close the lid; the lid unit body is designed to cover a portion of the manifold that is exposed from the case opening of the case; and the floating joint is arranged with its rotation axis being substantially parallel to the axis of rotation of the support unit of the lid unit body. a manifold including a sample chamber for housing the object to be measured, and further including an expansion chamber wherein the sample chamber comprises an opening and a circumferential edge region around the opening; and a lid unit including a lid unit body comprising a first end part and a second end part, and a non-rotary lid that is used for closing the opening of the sample chamber and is pressed against the circumferential edge region around the opening to hermetically seal the sample chamber; characterized in that the true density measurement device further comprises: a floating joint with a rotation axis by means of which the lid is swingably attached between the first end part and the second end part of the lid unit body; and a device case covering the manifold and pivotally supporting the lid unit body at its second end part through a support unit comprising an axis of rotation, wherein the device case comprises a case opening; wherein the lid unit body functions as an arm that is used for an operation to open or close the lid; the lid unit body is designed to cover a portion of the manifold that is exposed from the case opening of the case; and the floating joint is arranged with its rotation axis being substantially parallel to the axis of rotation of the support unit of the lid unit body. a floating joint with a rotation axis by means of which the lid is swingably attached between the first end part and the second end part of the lid unit body; and a device case covering the manifold and pivotally supporting the lid unit body at its second end part through a support unit comprising an axis of rotation, wherein the device case comprises a case opening; wherein the lid unit body functions as an arm that is used for an operation to open or close the lid; the lid unit body is designed to cover a portion of the manifold that is exposed from the case opening of the case; and the floating joint is arranged with its rotation axis being substantially parallel to the axis of rotation of the support unit of the lid unit body.