The electronic siren used on an ambulance is very loud in the street, but it is hard for the driver in an automobile to recognize it in many cases. It is conceivable that the reason is due to masking of the sounds inside and outside the automobile. In this paper, the detectable level of the electronic siren used on an ambulance was described and the reason why the thershold level of the siren is actually increased is discussed. As a result, conclusions were extracted as follows. (1) The threshold of the siren increases at the rate of 1.0--1.7 dB, when the level of music listened to in an automobile is increased by 2 dB in L[inf Aeq]. This is considered to be caused by masking. (2) In an automobile running at medium speed, the detectable level of the siren increases by about 0.5 dB compared to the case of standstill. This is concerned with attention to driving.