In this paper, the radiating element of 2x2 slots is designed for a 350 GHz-band corporate-feed waveguide slot array antenna using gold-coating silicon wafers with different thickness (30 μm, 180 μm, and 275 μm) to improve the bandwidth. A 20.0% bandwidth for the reflection less than -15 dB is achieved. The bandwidth for VSWR less than 1.5 in a 16x16-slot array with a feeding circuit is 18.7% by simulation. The aperture efficiency and the antenna efficiency at the center frequency of 362.5 GHz are 94.3% and 85.4%, respectively.