Bone fixation device for keeping two or more bone pieces together, either pieces of a broken bone or two distinct bones, for undergoing junction of the pieces by natural welding, the device being capable of immobilizing flat or round bones, long or short bones, parts of a broken femur or two contiguous vertebrae, the device comprising a fixation plate and fixation screws, the plate having orifices for passing the screws through the plate and fastening the screws into the bone tissue, a screw blocking or locking mechanism being provided in the plate to block the screws in the fastening position once the screws have been passed through the plate and screwed into the bone pieces, for preventing the screws from unscrewing from the bone pieces and moving out of the fixation plate once and after the fixation plate and the screws have been firmly installed in the bones, with the risk of entering into contact with the patient inner tissues, muscles or organs, the blocking mechanism being easily operated by a surgeon or a surgeon assistance.