The present invention relates to a security device for a syringe used in dentistry, consisting of a tubular sleeve inside which is provided a needle holder (2) axially movable between an advanced position wherein the needle (7) goes beyond the distal end of the tubular sleeve (1) and a retracted position wherein the needle (7) is fully retracted in the tubular sleeve (1). The tubular sleeve (1) has an internal section corresponding substantially to the external section of the syringe body (26), means (9, 30) for limiting the displacement of the needle holder (2) in the forward direction and at its rear part a second locking means (10, 36) providing for the passage of the needle holder (2) in a rearwards direction and preventing the return of the needle carrier into an advanced position.