Provided is a novel method for amplifying mass spectrometric signals. A novel method for detecting signals of a target molecule includes: allowing a sample, which comprises a target molecule, to contact a gold particle having a surface modified to selectively bind the target molecule, allowing a low molecular weight molecule engrafted to the gold particle generate mass spectrometric signals after the interaction, e.g., binding, between the gold particle and the target molecule, and amplifying the mass spectrometric signals to generate much mass spectrometric signals of the low molecular weight molecule even when trace amounts of the target molecule are present. An assay system using the method and the gold particle prepared in the method are provided. The method amplifies signals of the target molecule without pretreatment of a sample, making it possible to measure the target molecule simply and precisely.