PURPOSE: To provide a device for detecting irregular cutting of a band sawing maching which can surely and adaptively detect the generation of irregular cutting in cutting a material in a direct and non-contact manner, and preliminarily prevent the loss of the material attributable to the defective cut section.

CONSTITUTION: An optical system provided with a light emitting element 21 and a light receiving element 22 to receive the outputted light is mounted on supporting members 11, 12 of a band saw blade 8 in a band sawing machine so that the optical axis L may follow the cutting locus of the band saw blade 8 in the vicinity of the back part of the band saw blade 8, and a judging circuit 31 to input the output signal of the light receiving element 22 and judge the presence/absence of the irregular cutting from the size of the signal is provided.