A harmonica includes a comb with a plurality of cell sets each including alternately arranged first and second cells, a plurality of mouthpieces registered with the cell sets, a reed vibrating member having a plurality of reeds registered with the cells and configured to be vibrated by an air flow for producing a tone of either one of diatonic and chromatic scales, and a valve member with a plurality of valve units interposed between the mouthpieces and the cell sets and shiftable between diatonic and chromatic positions to block the air flow into the second and first cells, respectively, thereby allowing the player to play the notes of both diatonic and chromatic scales.