A signal masking circuit includes a detection circuit, a delayed read data strobe signal generation circuit, a gating circuit, a counting circuit, and a masking circuit. The detection circuit detects a period of a logic “L” of a read data strobe signal. The gating circuit gates a delayed read data strobe signal, and generates a first masked read data strobe signal. The counting circuit counts the falls of the first masked read data strobe signal until the count reaches a predetermined number, and generates a masking signal for masking the first masked read data strobe signal. The masking circuit masks the first masked read data strobe signal, and outputs a second masked read data strobe signal.