A stacked semiconductor device includes a first semiconductor device equipped with a first semiconductor chip 14 having a transistor circuit and protection diodes, and a second semiconductor device equipped with a second semiconductor chip 24 having a transistor circuit and protection diodes, and stacked on the first semiconductor device via a connection portion, wherein a power supply line connected to the first and second semiconductor chips is used in common, and a forward ON voltage of the protection diodes of the first semiconductor chip is set higher than a forward ON voltage of the protection diodes of the second semiconductor chip 24. When a connection test is executed, the forward ON voltage of the protection diodes of the first semiconductor chip or the second semiconductor chip is detected and then normal/open is judged.