When ultrashort laser pulses are focused inside a glass at a high repetition rate, structural changes occur because of the heat accumulation from the train of laser pulses. This report describes phase measurement of the structural changes induced inside glass by phase-shifting digital holographic microscopy. Two-dimensional phase distribution across the structural change for static exposure is retrieved. By focusing femtosecond laser pulses at the interface between two glass substrates, melted materials can directly weld glass plates. Quantitative phase measurement of welded glass substrates revealed that the refractive index decreased in the laser-irradiated zone.