Disk-shaped information recording medium, disk cartridge, and information recording/reproducing apparatus

A disk-shaped information recording medium having an outer diameter and an inner diameter, the information recording medium comprising:

• a substrate having a predetermined thickness;
• a first side that is one side of the substrate;
• a second side that is the other side of the substrate;
• a cylindrical portion forming a through hole formed at a center of the substrate;
• a thin portion formed so as to surround an outer diameter of the cylindrical portion, the thin portion being thinner than the predetermined thickness;
• a non-inclined surface formed at the thin portion;
• and
• an inclined surface formed on an outer diameter side of the non-inclined surface at the thin portion, the inclined surface inclined at an angle greater than or equal to 25 degrees and less than or equal to 35 degrees with respect to the non-inclined surface.