A platen opening mechanism in a portable labeling machine, comprising: a side plate having a bottom cover disposed in a pivotally openable manner; a retaining portion retaining a strip - form label wound into a roll at the side plate; and a platen enabling the label to be drawn out as a strip form from the retaining portion to be transferred onto a top surface thereof; the label being attachable to a member to be attached with, the platen being pivotable to a closed position and an open position relative to the side plate to allow the label to be loaded into the portable labeling machine, wherein the platen is put in an open state relative to the side plate in accompaniment with an operation of opening the bottom cover from the side plate, wherein the bottom cover is formed with a hook - shaped platen-opening engagement/ disengagement portion which is configured to protrude toward a flange portion of the platen, and the platen-opening engagement/disengagement portion is engageable with and disengageable from the flange portion protrudingly formed on the platen, wherein the platen-opening engagement/disengagement portion is formed confronting the flange portion on a side of a opening/closing shaft of the bottom cover and has at its interior portion an under-surface engagement/disengagement surface and at its top surface side an engagement/disengagement tapered surface, and wherein the open position of the platen is adjustable by selecting a length of a flange portion of the platen and wherein the platen is allowed to open and close among the closed position, a first open position and a second open position, wherein the first open position is when the platen is substantially half opened from the side plate, as a result of an operation of opening the bottom cover, and the second open position is when the platen is fully opened from the side plate, as a result of an operation of further opening the platen itself from the first open position. a side plate having a bottom cover disposed in a pivotally openable manner; a retaining portion retaining a strip - form label wound into a roll at the side plate; and a platen enabling the label to be drawn out as a strip form from the retaining portion to be transferred onto a top surface thereof; the label being attachable to a member to be attached with, the platen being pivotable to a closed position and an open position relative to the side plate to allow the label to be loaded into the portable labeling machine, wherein the platen is put in an open state relative to the side plate in accompaniment with an operation of opening the bottom cover from the side plate, wherein the bottom cover is formed with a hook - shaped platen-opening engagement/ disengagement portion which is configured to protrude toward a flange portion of the platen, and the platen-opening engagement/disengagement portion is engageable with and disengageable from the flange portion protrudingly formed on the platen, wherein the platen-opening engagement/disengagement portion is formed confronting the flange portion on a side of a opening/closing shaft of the bottom cover and has at its interior portion an under-surface engagement/disengagement surface and at its top surface side an engagement/disengagement tapered surface, and wherein the open position of the platen is adjustable by selecting a length of a flange portion of the platen and wherein the platen is allowed to open and close among the closed position, a first open position and a second open position, wherein the first open position is when the platen is substantially half opened from the side plate, as a result of an operation of opening the bottom cover, and the second open position is when the platen is fully opened from the side plate, as a result of an operation of further opening the platen itself from the first open position.