A printer comprising: a printer mechanism unit including a frame and a print head; a platen unit including a platen roller ; a detection switch configured to detect connecting of the platen unit to the printer mechanism unit ; and a switch holder having the detection switch attached thereto and covering part of the detection switch , wherein: the switch holder is mounted to one of the frame and the platen unit ; characterised in that a first internal face of the switch holder has a rib projecting from the first internal face with an arc-shaped cross-section, and the detection switch is fastened inside the switch holder between the rib on the first internal face and a second internal face of the switch holder opposite to the first internal face, wherein the rib is in line contact or point contact with the detection switch ; the detection switch is connected to a flexible substrate , and the switch holder has one or more holder projections ; and the flexible substrate has one or more securing holes corresponding in position to the one or more holder projections , and the one or more holder projections are configured to enter the one or more securing holes , respectively, of the flexible substrate when the detection switch is attached to the switch holder . a printer mechanism unit including a frame and a print head; a platen unit including a platen roller ; a detection switch configured to detect connecting of the platen unit to the printer mechanism unit ; and a switch holder having the detection switch attached thereto and covering part of the detection switch , wherein: the switch holder is mounted to one of the frame and the platen unit ; characterised in that a first internal face of the switch holder has a rib projecting from the first internal face with an arc-shaped cross-section, and the detection switch is fastened inside the switch holder between the rib on the first internal face and a second internal face of the switch holder opposite to the first internal face, wherein the rib is in line contact or point contact with the detection switch ; the detection switch is connected to a flexible substrate , and the switch holder has one or more holder projections ; and the flexible substrate has one or more securing holes corresponding in position to the one or more holder projections , and the one or more holder projections are configured to enter the one or more securing holes , respectively, of the flexible substrate when the detection switch is attached to the switch holder . the switch holder is mounted to one of the frame and the platen unit ; characterised in that a first internal face of the switch holder has a rib projecting from the first internal face with an arc-shaped cross-section, and the detection switch is fastened inside the switch holder between the rib on the first internal face and a second internal face of the switch holder opposite to the first internal face, wherein the rib is in line contact or point contact with the detection switch ; the detection switch is connected to a flexible substrate , and the switch holder has one or more holder projections ; and the flexible substrate has one or more securing holes corresponding in position to the one or more holder projections , and the one or more holder projections are configured to enter the one or more securing holes , respectively, of the flexible substrate when the detection switch is attached to the switch holder . <EOS>