When an acoustic wave passes through short narrow channels in a regenerator having an axial temperature difference, the acoustic power is amplified for the waves going from cold to hot, whereas it is damped for the waves going in the opposite direction. This study applies such asymmetric wave propagation to demonstrate a thermoacoustic diode, which plays the role of the acoustic counterpart to an optical isolator. Four regenerators having the same longitudinal temperature difference are aligned in series to make four-stage amplification and damping of the acoustic power possible. This alignment leads to the enlarged difference between the acoustic power gains in the forward and backward propagation directions, even with a moderate temperature difference. Furthermore, by introducing the acoustical impedance-matching unit, the power-reflection coefficient is kept as low as 0.017 in forward propagation. The results show that the power-transmission coefficients in the forward and backward directions, respectively, reach 0.98 and 0.023, which means that the power-transmission ratio is 16 dB.