The invention provides a belt monitoring device for an injection molding machine. The belt monitoring device comprises rotary encoders (7a and 7b) detecting rotation positions of rotation driving portions (4a and 4b), a deviation calculating means (8) that obtains a deviation (Ke) between an encoder output value (Da) outputted from one specific rotary encoder (7a) and an encoder output value (Db) outputted from the other rotary encoder (7b), an abnormality determining means (9) that compares the deviation (Ke) with threshold values (Ksf and Kss) set in advance and determines that the deviation (Ke) has become the threshold value (Ksf) or more and/or the threshold value (Kss) or more, and an abnormality output means (10) that outputs at least a determination result of the abnormality determining means (9).