A three-dimension active silencer includes a measuring unit for detecting a signal relating to a sound radiated from a target sound source which is located in a closed box. A plurality of additional sound sources are arranged around the target sound source. A microphone is arranged at a predetermined position for detecting a sound power at the position. A controller is adapted to control respective amplitudes of the additional sound sources which can be different from an amplitude of the target sound source and respective phases of the additional sound sources, based on the signal detected by the measuring unit, in such a manner that the microphone detects the lowest sound power. The additional sound sources are arranged in such a manner that a position xp of the target sound source, respective relative positions di (i=1, 2, . . . , N) of the N additional sound sources from the target sound source and a wave number k of the sound radiated from the target sound source can substantially satisfy the following expression. 1 N ⁢ ∑ i = 1 N ⁢   ⁢ cos ⁢   ⁢ k ⁡ ( x p - d i ) - cos ⁢   ⁢ k · x p ≅ 0 According to the feature, it can be achieved to reduce the total sound power in the closed box including the target sound source.