Provided is a cutter that is applicable to a cutting device with variable structures and that allows exchange of a cutting blade safely and smoothly with a simple structure. The present invention provides a cutter comprising: a slider; a holder member that retains a cutting blade; and a removing-and-installing member that removes or installs the holder member from or into the slider in a condition that the cutting blade is shielded with a cover member, wherein the cover member is provided with a projection portion while the holder member is provided with a groove portion, and when the removing-and-installing member is installed into the slider along with the holder member, the projection portion is adapted to travel in the groove portion so as to expose a cutting edge of the cutting blade through a cutout.