We found “normal-like” abnormal records observed with STS-2 broadband seismometer. The anomalous records have the three-dimensional particle motion on a certain plane, which might be due to some instrumental failures. We also proposed a monitoring index based on the scalar triple product of three unit vectors sampled from the three-component records with regular time intervals to detect the abnormal waveforms. The value of the index becomes zero when the particle motion lies only on a plane or a line. We applied the proposed method to STS-2 records of one JMA station and one F-net station (NIED), and confirmed that our method gives us a straightforward way to identify the anomalous records of the STS-2.