We report a simple subwavelength-diameter plastic wire, similar to an optical fiber, for guiding terahertz wave with a low attenuation constant. With a large wavelength-to-fiber-core ratio, the fractional power delivered inside the lossy core is reduced, thus lowering the effective fiber attenuation constant. In our experiment, we adopt a polyethylene fiber with a 200-500 μm diameter for guiding terahertz waves in the frequency range of 0.2-0.5 THz in which the attenuation constant is reduced to the order of or less than 0.01 cm-1. Direct free-space coupling efficiency, as high as 20%, can be achieved by using an off-axis parabolic mirror. Furthermore, all the plastic wires are easily available in our daily life without complex processes and expensive costs.