An apparatus for detecting an on-off keying signal in a CDMA mobile station device is disclosed. A first calculator receives an on-off keying signal and a reference signal transmitted from a base station transmitter at a given transmission power level, despreads the received on-off keying signal, and generates a symbol power level of the despread on-off keying signal. A second calculator despreads the reference signal and generates a power level of the despread reference signal. A multiplier generates a variable threshold value by multiplying the power level of the despread reference signal by a gain determined by a ratio of the transmission power of the reference signal from the base station transmitter to the transmission power of the on-off keying signal from the base station transmitter. A comparator compares the variable threshold value with the symbol power level of the despread on-off keying signal. Further, a symbol detector detects the symbol of the on-off keying signal by comparing a comparison value from the comparator with a prescribed threshold value.