In a multilayer substrate, ground electrodes are spaced apart from each other in a thickness direction of a laminate substrate and are adjacent to signal lines in a width direction of the signal lines. In a first transmission line region in which a maximum line width of the signal lines is smallest, the signal lines do not overlap any of the ground electrodes in the thickness direction of the laminate substrate. In a second transmission line region in which the maximum line width of the signal lines is largest, one ground electrode among the ground electrodes overlaps the signal lines other than the signal line adjacent to the one ground electrode among the signal lines.