Tachikawa fault is an active fault located in the western part of the Tokyo Metropolitan area. A structure of deep sedimentary layers around the fault was explored by microtremor array measurements and an analysis of existing seismic refraction data for the purpose of estimating its effects on earthquake ground motion. It was found from the microtremor array measurements that a basement depth in S-wave profile at down-thrown side of the fault is larger than that at up-thrown side with depth difference of 1.6km. A two-dimensional model was constructed using the explosion data and the depth data on the sediments derived from the microtremor array measurements. Travel times of initial P-wave for the proposed model calculated by a finite difference ray tracing fit the observed travel times better than the model from a conventional time term analysis. We evaluated spatial variations of amplification of earthquake ground motion in the proposed model using a 2D finite difference numerical simulation. It was found that ground motion in the down-thrown side is amplified due to effects of the basement step.