To provide a device for detecting an imperfect package which detects a slip or imperfect bite of the package pack by using a displacement sensor at longitudinal and transverse heat-sealing.

The device is provided with a displacement sensor 4 for longitudinal seal integrally provided at a front side heater 2a for longitudinal seal and a rear side bracket 3b integrally provided at a rear side heater 2b for longitudinal seal of a longitudinal seal device 2, and a displacement sensor 8 for transverse seal integrally provided at a front side heater 6a for transverse seal and a rear side bracket 7b integrally provided at a rear side heater 6b for transverse seal of a transverse seal device 6, a control part 10 determins the presence of imperfect superposing of a superposed end portion 1a of a film 1 by calculating a gap g2 between the heaters 2a and 2b for the longitudinal seal comparing with a predetermined value f2 based on the output from the sensor 4, and determins the presence of imperfect bite of a packaged body 5 to a transverse seal portion 1d by calculating a gap g6 between the heaters 6a and 6b for the lateral seal comparing with a predetermined value f6 based on the output from the sensor 8.