A paper folding device that is configured to fold a planar paper sheet while conveying the paper sheet along a paper sheet conveyance surface, the paper folding device comprising: a pair of first conveyance rollers disposed on an upstream side in a paper sheet conveyance direction; a pair of second conveyance rollers disposed on a downstream side in the paper sheet conveyance direction; a switching gate plate installed on the paper sheet conveyance surface between both pairs of the rollers; a folding plate disposed above the switching gate plate; and a control unit that is configured to control an operation of an entire device, wherein the switching gate plate is switchable between a first guide position at which the switching gate plate is configured to interfere with the conveyed paper sheet and guide a leading end portion of the paper sheet upward so as to pass through an upper surface of an upper roller of the pair of second conveyance rollers and a second guide position at which the switching gate plate is configured to guide the leading end portion of the paper sheet toward a nip portion of the pair of second conveyance rollers along the paper sheet conveyance surface, the folding plate is configured to be movable in a vertical direction in a substantially horizontal state, and is switchable between a folding position close to the switching gate plate disposed at the second guide position and a retracting position retracted at a predetermined interval from the switching gate plate, and the control unit is configured to perform control to, after guiding the leading end portion of the paper sheet to be conveyed by the switching gate plate disposed at the first guide position so as to pass through the upper surface of the upper roller of the pair of second conveyance rollers by a predetermined amount, switch the switching gate plate to the second guide position, and then lower the folding plate disposed at the retracting position to the folding position in a substantially horizontal state so as to sandwich the paper sheet on the switching gate plate, thereby folding back the leading end portion of the paper sheet along a surface of the upper roller of the pair of second conveyance rollers. a pair of first conveyance rollers disposed on an upstream side in a paper sheet conveyance direction; a pair of second conveyance rollers disposed on a downstream side in the paper sheet conveyance direction; a switching gate plate installed on the paper sheet conveyance surface between both pairs of the rollers; a folding plate disposed above the switching gate plate; and a control unit that is configured to control an operation of an entire device, wherein the switching gate plate is switchable between a first guide position at which the switching gate plate is configured to interfere with the conveyed paper sheet and guide a leading end portion of the paper sheet upward so as to pass through an upper surface of an upper roller of the pair of second conveyance rollers and a second guide position at which the switching gate plate is configured to guide the leading end portion of the paper sheet toward a nip portion of the pair of second conveyance rollers along the paper sheet conveyance surface, the folding plate is configured to be movable in a vertical direction in a substantially horizontal state, and is switchable between a folding position close to the switching gate plate disposed at the second guide position and a retracting position retracted at a predetermined interval from the switching gate plate, and the control unit is configured to perform control to, after guiding the leading end portion of the paper sheet to be conveyed by the switching gate plate disposed at the first guide position so as to pass through the upper surface of the upper roller of the pair of second conveyance rollers by a predetermined amount, switch the switching gate plate to the second guide position, and then lower the folding plate disposed at the retracting position to the folding position in a substantially horizontal state so as to sandwich the paper sheet on the switching gate plate, thereby folding back the leading end portion of the paper sheet along a surface of the upper roller of the pair of second conveyance rollers.