To obtain an inspection device for accurately and securely detecting the internal cavity of a casting.

External force is applied to a casting 1 with a hammer 2, the external pressure when a hammering sound is generated is detected by a vibration force sensor 2, and the ratio to an external force reference value is obtained by a ratio operation unit 15. Further, a hammering sound being generated in the casting 1 is detected by a microphone 3, is separated into a time series signal for each frequency band by an analog BPF 5, and is further rectified, thus extracting the instantaneous maximum value at each frequency band by a peak hold 7. The maximum value is corrected based on a ratio that is calculated by the ratio operation unit 15 and is compared with a preset sound pressure reference value by a comparator 9, a comparison result signal is outputted when the comparison result deviates from a specific relationship, and an alarm 18 gives a warning when the number of comparison result signals is, for example, equal to or more than three. The maximum value of the time series signal for each of a plurality of frequency bands is extracted for comparison, thus improving judgment reliability.