An injection device (1) for injecting medical substances and designed to be implanted in a subcutaneous zone of the body of a patient. The device includes a hollow rigid housing (2) having an opening which is closed hermetically by a membrane (3) so as to define a chamber (12). A duct (4) is connected to the housing to connect the chamber to the outside of the device. The device also has a casing (15) of elastomer material which covers the housing, and the membrane forms an integral portion of the casing. The implantable device is suitable for performing chemotherapy treatment.