Flow velocity sensor and a length sensor are provided on or near an edge of the lip plate which the air jet from the embouchure hole impinges against. Jet flow velocity Ue at the edge and a jet-blowout-outlet-to-edge distance d are detected by the sensors. Jet transfer time .tau.e is calculated by an equation of .tau.e=d/Ue, and a jet traveling angle .theta.e' is calculated by an equation of .theta.e'=2.pi.fso1.times..tau.e (where fso1 represents a frequency of a tone to be generated). When .theta.e' has decreased to .pi./2 during tone generation in a primary mode, the mode changes to a secondary mode to raise the pitch of the currently generated tone by one octave. When .theta.e' has increased to 3.pi./4 during tone generation in the secondary mode, the mode changes to the primary mode to lower the pitch of the currently generated tone by one octave.