In this presentation, we report on the phase-randomizing functionality of a new absorber (microcoil absorber) consisting of metal-plated microcoils of spiral alga Spirulina dispersed in a low-reflective-index material. We fabricated samples of the absorber in which microcoils dispersed in a polystyrene foam and measured the temporal waveforms of the normal reflection by terahertz time-domain spectroscopy (THzTDS). Then, it was confirmed that the reflected waveform consisted of many small pulses and that the phase of the reflected waveform was randomized.