A prepreg is used to fabricate a semiconductor package including a chip and a substrate to mount the chip thereon. The prepreg is in a semi-cured state. The substrate includes a cured product of the prepreg. The chip has: a first chip surface located opposite from the substrate; and a second chip surface located opposite from the first chip surface. The prepreg satisfies the relational expression: 0.9≤X2/X1≤1.0 (I), where X1 is a coefficient of thermal expansion of the first chip surface of the chip before the chip is mounted on the substrate, and X2 is a coefficient of thermal expansion of the first chip surface of the chip after the chip has been mounted on the substrate.