To correctly detect lack in the number of pieces, for example, of cigarette packs in a carton or a parcel.

A lack-of-piece detector includes a first proximity sensors 28 for detecting a cigarette packs Cp at both ends in a carton and a second proximity sensors 32 placed immediately above the inspected carton side by side which can detect the cigarette packs Cp except those at both ends in the carton wherein both sensors are facing across the carton when the carton reaches an inspection position, and the second proximity sensors 32 are placed with a distance X larger than a width Y of the cigarette pack Cp.