A reflective signal booster for omni-directional antenna is applicable to a tube or flat omni-directional dipole antenna for changing the antenna from an omni-directional type into a directional type, preventing interference of the radiation of the antenna with other equipments, and improving the antenna gain. The signal booster which can be attached easily to an antenna is composed of a reflector formed by two metal plates forming an included angle that is fixed by an angle fixer. The antenna is supported by a supported member and thus properly located between the metal plates. Thus, electromagnetic waves from the antenna and traveling toward the metal plates is blocked and reflected toward a front opening formed between the metal plates whereby signal intensity in a frontward direction is enhanced and interference with equipments located behind the metal plates is eliminated.