According to one embodiment, a semiconductor device includes an ECC decoder which performs diagnosis on data using an error detection code for the data, an ECC encoder which generates an error detection code for a first data piece equivalent to a bit range accounting for a part of plural bits configuring the data and generates an error detection code for a second data piece equivalent to a bit range accounting for a remaining part of the bits, and a diagnosis circuit which, when no error in the data has been detected by the ECC decoder, compares a part of the data corresponding to the first data piece with the first data piece used in generating the first error detection code and compares a part of the data corresponding to the second data piece with the second data piece used in generating the second error detection code.