A matrix-assisted laser desorption (MALDI) mass spectrometry in which a mixture comprising a polyethylene glycol derivative having a molecular weight of 40,000 or more and a matrix is irradiated with laser beam to vaporize the polyethylene glycol derivative together with the matrix, thereby measuring a molecular weight of the polyethylene glycol derivative, wherein as the matrix, a low polar matrix having a 1-octanol/water partition coefficient (logPow) of 4.1 to 4.8 is used. Also disclosed is a matrix for use in the matrix-assisted laser desorption (MALDI) mass spectrometry.