A semiconductor device includes a relay chip configured to be connected to a host; a first chip connected to the relay chip via a first channel; and a second chip connected to the relay chip via a second channel. The relay chip is configured to receive, from the host, a first enable signal for selecting the first channel and a second enable signal for selecting the second channel. During a first period in which the first enable signal is maintained at a non-active level and the second enable signal is maintained at an active level, the relay chip is configured to perform, in parallel, a first data transfer operation via the first channel and a first command issuing operation via the second channel.