We propose, theoretically analyze and experimentally demonstrate a novel broadband operating Si-nanowire multistage delayed Mach-Zehnder interferometer (DMZI) based (De)MUXs utilizing wavelength insensitive couplers (WINCs) and the discrete phase shifters for the phase matching between cascade-connected DMZIs. Based on the coupled mode theory and transfer matrix method, the coupling characteristics of a Si-wire directional coupler (DC) and WINC are analytically discussed from the viewpoint of a wavelength sensitivity of coupling efficiency [κCoup(λ)]. We theoretically verify that the operating window of the proposed DeMUX can be as broad as >110 nm by introducing the WINC and the additional phase matching between multiple DMZIs. Based on the theoretical analysis, 300-mm waferscale ArF-immersion lithography process are used to fabricate the Si-wire-based 1×4Ch DeMUXs. It will be shown that >110-nm-wide operating window for the proposed DeMUXs with four kinds of channel spacings (Δν=400, 800, 1250, 1900GHz). The theoretically identified aspect of nearly constant κCoup(λ) by the WINC and the phase matching by the phase shifter is proven to be extremely effective way to make the production yield much better, because DeMUX spectral response keeps nearly constant even if insufficient fabrication accuracy makes spectral response be shifted toward longer or shorter wavelength side. In addition, it will be shown that operating spectral window could be made much wider by optimizing the WINC design parameters in the DeMUX configuration. The proposed scheme would be attractive for increasing available channel count without inducing any excessive losses, which makes the proposed scheme more practical in WDM optical transceivers.