This paper presents a prototype of a 3D imaging radar system at 10-20GHz for the non-destructive inspection of wooden house walls. We called the radar system as a multi-path linear array radar (MPLAR) system. The prototype of MPLAR system is consisted mainly of a data processing unit and a radar unit. The data processing unit is composed of a laptop PC, an A/D converter and a battery. The radar unit is composed of one-dimensional array antenna (32 transmitting and 32 receiving antennas which are resistively loaded printed bowtie antennas), two high speed RF switch units (SP32T × 2), a broadband step frequency radar unit (10-20GHz), a digital switching control unit and a radar control unit. The system acquires the frequency domain data for all the transmitting and receiving antenna pairs, i.e. 32 × 32 = 1024 pairs, in 33 milliseconds at a position. The number of frequency step is 64. So the step frequency is (20GHz-10GHz) / 64 = 156.25MHz. In two experiments, we indicated that the prototype of MPLAR system can output the 3D image of a wooden brace behind a decorated gypsum board of a Japanese wooden house wall by using synthetic aperture radar (SAR) technique. The SAR processing time, the measurement speed and the weight of the radar unit resulted in 3.6sec at 512mm measurement, 6cm/sec and 8kg respectively.