This paper proposes a hydraulic inerter–damper inspired by the idea of the “mass pump” developed by Kawamata et al. . A mass pump produces apparent mass that is approximately two thousand times larger than the physical mass of the fluid that can fill a thin tube. This device is one of the implementations of the concept of an inerter defined by Smith . The most significant features of an inerter are that it produces inertial resistive force that is proportional to the relative acceleration between two nodes, and its apparent mass is substantially larger than its physical mass. Kawamata and Onuma  further developed a tuned mass damper–like system using the apparent mass produced by the inerter–device. The basic concept of a fluid inerter invented by Tuluie  is the same as that of a mass pump. Wang et al.  developed a hydraulic inerter to obtain an amplified apparent mass by combining a flywheel and a hydraulic gerotor. The maximum estimated inertance (apparent mass) of the device in the experiment was 687 kg. The hydraulic inerter–damper proposed in this paper combines the mechanism used in the hydraulic inerter developed by Wang et al.  and a sub-piston with orifices, generating inertial resistive and fluid viscous damping forces, respectively. Dynamic testing using a full-scale hydraulic inerter-damper confirmed that the mass of the flywheel attached to the gerotor led to an apparent mass of 12 014 metric tons.