To prevent a rotary blade, a motor and a main circuit section constituting a driving circuit of the motor in a letter opener from being exposed outside on the occasion of replacement of a battery.

A rotary blade body 15 which cuts the edge part La of an envelope L inserted into a passing groove 10a and moved along this groove 10a, along the direction of the movement, and the motor 16a which drives the rotary blade body 15 are housed in the internal space 13 of an upper box 1, while the main circuit section 16 in the driving circuit of the motor 16a is provided therein. In a lower case 2, a battery loading part 22 is formed and also a supply circuit section 23 in the driving circuit of the motor 16a is provided therein. By fitting the lower case 2 to the upper box 1, the main circuit section 16 and the supply circuit section 23 are connected together electrically.