Infinite polyacenacene has been studied by the same method as applied by Salem and Longuet-Higgins (1960) to infinite polyacetylene and polyacene. Polyacenacene can be considered as three polyacetylene chains joined by cross links. It is predicted that the most stable configuration is the equilibrium configuration exhibiting glide plane symmetry, in which the C-C bonds of three chains all have the same length, being slightly shorter than the cross bonds. This stable configuration of equilibrium has zero gap and zero slope at the Fermi level.