The present invention is a polishing agent for a synthetic quartz glass substrate, containing polishing abrasive grains, a polishing accelerator, and water, wherein the polishing abrasive grains are wet ceria particles, and the polishing accelerator is a polyphosphoric acid, a salt thereof, a metaphosphoric acid, a salt thereof, a tungstic acid, or a salt thereof. There can be provided a polishing agent for a synthetic quartz glass substrate that has high polishing rate and can sufficiently reduce generation of defects due to polishing.