To provide a biaxial in-plane compression test method capable of generating a compression strain of 2.5% or more while reducing stress variations in a material test in which a cross-shaped thin plate test piece is subjected to a biaxial compression load.SOLUTION: The biaxial in-plane compression test method is for performing a material test by applying a biaxial compression load to a cross-shaped thin plate test piece 1. The thin plate test piece 1 has a measurement part 3, circular notch parts 7 formed by notching each corner part of the cross shape in a circular shape, and a plurality of hole-shaped portions 9 provided on each line connecting the centers O of the circular notch parts 7 adjacent to each other surrounding the measurement part 3. The biaxial compression load is applied to the measurement part 3 via the two pairs of opposing pieces 5a and 5b extending from the measurement part 3.SELECTED DRAWING: Figure 1
