A gas chromatograph (GC) apparatus capable of detecting abnormality, in which a reference retention index is obtained from a result obtained by analyzing a given substance and a retention-index reference substance under adequate analysis conditions using a normal column, and stored in a storage section. Further, under a condition that the apparatus can perform normal analysis, the given substance is subjected to GC analysis to obtain a reference retention time, and the obtained reference retention time is stored in the storage section. A diagnosis processing section is operable to compare each of an actual retention time and an actual retention index with a respective one of the reference retention time and the reference retention index. If a deviation therebetween or abnormality is detected, the diagnosis processing section is operable to estimate a causal factor of the abnormality.