A flag signal generation circuit includes a first periodic signal detection unit, a second periodic signal detection unit, and a flag signal output unit. The first periodic signal detection unit is configured to detect a change in a level of a first periodic signal and generate a first detection signal. The second periodic signal detection unit is configured to detect a change in a level of a second periodic signal and generate a second detection signal. The flag signal output unit is configured to generate a pre-flag signal from the first and second detection signals, buffer the pre-flag signal in response to a mode register read signal, and output the buffered pre-flag signal as a flag signal.