The storage of living organisms in alginate gel fibers of various shapes is critical for various applications, and has, therefore, attracted research attention in the field of regenerative medicine. This paper presents a report on the on-demand formation of heterogeneous alginate gel fibers using a microfluidic device with narrowly spaced, two-dimensional array of microapertures. The device was fabricated by bonding stainless-steel substrates that formed an array of 48 microapertures individually connected to microchannels. This configuration allowed for the independent control of fluid injected from the microapertures at a narrow spacing of 50 μm, and the formation of gel fibers of various shapes and dimensions with a single device. Using this stainless device, a single-component type gel was formed by setting one aperture for Na-alginate injection at the center of the aperture array; the surrounding apertures were set for CaCl2 injection; the fiber diameter ranged from 8 to 28 μm. In addition, two-component, staggered-grid, hollow, and asymmetric gel fibers were formed by designing apertures for fluid injection. In the staggered-grid type formation, the cross-sectional area of the fiber was controlled between 6.0 and 12.0 × 104 μm2 by changing the CaCl2 injection flow rate. This adaptability to the formation of various two-dimensional heterogeneous gel fibers with a single device can promote the development of three-dimensional cultures and regenerative medicine based on the gel fiber technique.