An earplug unit includes a signal processing unit. The earplug unit is inserted into an external auditory canal and then used. In the earplug unit, a speaker is arranged at an eardrum side, and a microphone is arranged at a side opposite to the eardrum side. The microphone collects a sound in which a user's head shape or auricle characteristic is reflected. An output signal of the microphone is amplification-processed by a volume adjusting unit and then supplied to the speaker. An amplified sound is output from the speaker. A frequency characteristic is smoothed in a band larger than at least an audible frequency band in a system from the microphone to the speaker.