A rule was derived for constructing the unit element of a static micromixer that performs multiplexing of fluid layers by combining splitting, 180 degrees rotation and recombining-superposition in 3D bent channels. Based on the rule, a Y-type plate static mixer was developed, which consists of a combination of a Y-branch, 120 degrees and 60 degrees bent channels with rectangular cross-section and circular channels interconnecting these rectangular channels. The number of unit elements required for complete mixing 'n' was measured for a Y-type static mixer with square cross-section by conducting a decolorizing reaction with iodine. In the case of low Reynolds numbers, the element number it increased with Re. By modeling the mixing process in a fluid layer whose thickness reduces by half after the fluid passes through each unit element, a functional relation was derived to express it, based on which the experimental results for n at small Re were correlated. In the case of high Reynolds number, n decreased with Re, and the behavior was correlated experimentally with the variables in the functional relation. CFD analysis was conducted at Reynolds number up to 50. The results at Re=50 showed that fluid layers are deformed largely by secondary flows generated in the 3D bent channels, which promote mixing rate and decrease n with Re. Effects of the aspect ratio, a/b, of the rectangular cross-section of the mixer were investigated by CFD calculation. Deformation of the fluid interface is large for a/b >= 2.0, and splitting does not progress uniformly in the branch channel for a/b <= 0.2. Hence it is recommended to adopt channels whose cross-section is as close to a square as possible for promoting ideal splitting and recombination of fluid layers in the plate static mixer developed in this study.