The concept of acoustic impedance was known in the era of 1910 and its clinical application has been done after Metz's report in 1946. Thereafter, many studies on the impedance have been performed vigorously and hearing test by impedance audiometry has held important position in the field of audiometry. Recently author performed acoustic reflex and reflex decay test on 88 normal ears and 56 ears with unilateral cochlear deafness. The results are as follows ; 1) It normal ears, the average values of acoustic acostic reflex sensation level were 77.3 dB in 500 Hz, 74.5 dB in 1000 Hz, 79.7 dB in 2000 Hz and 80.1 dB in 4000 Hz in crossed reflex and 81.5 dB in 1000 Hz and 81.7 dB in 2000 Hz in uncrossed reflex, giving the result of more high value in uncrossed reflex, i.e; 7 dB and 2 dB more high value in 1000 Hz respectively. 2) The absence of acoustic reflex in normal ears were 9.1% 6.8%, 5.7% & 19.3% in 500 Hz, 100 Hz, 2000 Hz & 4000 Hz respectively, showing low incidence of acoustic reflex in 4000 Hz and comparing crossed with uncrossed reflex there were 11.4% & 17.0% respectively, showing relatively low incidence of acoustic reflex. 3) The incidence of acoustic reflex decay in normal ears in crossed reflex were 2.3%, 0%, 4.6% & 36.4% in 500 Hz, 1000 Hz, 2000 Hz & 4000 Hz respectively, showing marked increase in reflex decay in 4000 Hz and 4.5 & 9.0% in 1000 Hz & 2000 Hz respectively, showing relatively high incidence of reflex decay in 2000 Hz. 4) In cochlear deafness, the acoustic reflex sensation level was decreasing tendency as the hearing loss became severer in crossed reflex and similar result was seen in crossed reflex. There were no significant difference in crossed reflex sensation level and uncrossed reflex sensation level. 5) The frequency of acoustic reflex in cochler deafness became lower as the hearing loss became severer. There was marked reductionin frequency of acoustic reflex in 4000 Hz than in other frequency. The incidence of decay in uncrossed reflex was lower than that in crossed reflex. 6) The types of acoustic reflex in cochlear deafness were diagonal in 66%, normal in 9%, inverted L in 9%, horizontal in 2% and others in 14%.