A digital filter imparting a desired tone color to the tone signal is realized by the combination of a digital pole filter and a digital zero filter capable of controlling a pole and a zero in an amplitude-frequency characteristic respectively. The amplitude-frequency characteristic of the digital filter is a composite of the respective characteristics of the pole filter and the zero filter. A "peak" portion and a "valley" portion in this composite characteristic can be set independently by the pole control of the pole filter and the zero control of the zero filter, whereby the desired tone color characteristic can be readily realized. Further, the digital filter may comprise a plurality of digital filters having respective different constructions and a connection switching circuit capable of setting arbitrarily connection between these filters. The filter characteristic as a whole can be diversely altered by changing the connection, whereby diverse tone color characteristics can be realized with ease.