To provide a sample preparation device that is appropriate for the formation of a matrix film for MALDI through vacuum vapor deposition. A sample preparation device is provided with: a sample substrate support unit 23 for supporting a substance to be analyzed on a substrate S so that the substance faces a vapor deposition source 21 for a matrix substance J; a light amount measurement unit for irradiating a matrix film vapor deposited on the substrate S with measurement light diagonally and detecting the amount of measurement light that has transmitted through or has been reflected from the above-described matrix film diagonally; and an adhesion prevention means 23a for preventing the matrix substance that has flown off from the above-described vapor deposition source 21 from adhering to the above-described light amount measurement unit.