Abstract: In a method for generating a terahertz electromagnetic wave by using coherent phonons in a quantum structure, high density of coherent phonons acting as a radiation source of the terahertz electromagnetic wave are obtained by exciting the inter-band transitions in a quantum structure under the condition that the energy difference between two inter-band transitions in the quantum structure is resonated with the energy of the coherent phonons, and the terahertz electromagnetic wave is generated by the oscillatory polarizations of the coherent phonons. High density of coherent phonons may be created through the elementary excitation generated by using instantaneous laser pulses. A quantum well structure is effective as the quantum structure, and a scattering process of coherent phonon may be suppressed by confining coherent LO phonons in a well layer.