A newly developed lens positioner and an analogue-to-digital (AD) board are used in a low-cost sub-THz spectroscopic system with a multimode laser diode and photoconductive antennas (PAs). Among PAs, the spiral-type ones are preferable for the system, because they have high efficiency for emission and detection of the sub-THz radiation. However, positioning of the focusing lens requires high accuracy (~1μm) and then it takes much time. The new lens positioner utilizes leverage in a commercially available mirror holder and allows for more precise adjustment than the conventional one. In addition, the system employs a lock-in amplifier, which is rather expensive. The AD board costs about one third of the lock-in amplifier and enables cost reduction of the system. Further, the signal-to-noise ratio is improved by about 1.4 times and the measurement time can be reduced to half.