A quick coupler circuit (20, 30) for attaching an attachment to a quick coupler and detaching the attachment from the quick coupler, the quick coupler circuit (20, 30) comprising: a coupler cylinder (21, 30) configured to be driven due to the supply of hydraulic fluid, in a locking direction to lock the attachment to the quick coupler and in an unlocking direction to unlock the attachment from the quick coupler; an actuator configured to be driven due to the supply of hydraulic fluid; a first hydraulic pump (23, 39) configured to be connected in parallel with the coupler cylinder (21, 31) and the actuator, and configured to supply the hydraulic fluid to the coupler cylinder (21, 31) and the actuator; a boosting valve (25, 35) configured to switch between a boosting position to raise a pressure of the hydraulic fluid supplied from the first hydraulic pump (23, 39) to the coupler cylinder (21, 31), and a non-boosting position not to change the pressure of the hydraulic fluid supplied from the first hydraulic pump (23, 39) to the coupler cylinder (21, 31); a coupler changeover valve (27, 37) configured to switch between a lock-side position to supply hydraulic fluid from the first hydraulic pump (23, 39) to the coupler cylinder (21, 31) so as to drive the coupler cylinder (21, 31) in the locking direction, and an unlock-side position to supply the hydraulic fluid from the first hydraulic pump (23, 39) to the coupler cylinder (21, 31) so as to drive the coupler cylinder (21, 31) in the unlocking direction; and a changeover switch (28, 38) configured to switch between an unlock position, a lock position, and a hold position, wherein when the changeover switch (28, 38) switches to the lock position, the boosting valve (25, 35) switches to the boosting position and the coupler changeover valve (27, 37) switches to the lock-side position; and when the changeover switch (28, 38) switches to the unlock position, the boosting valve (25, 35) switches to the boosting position and the coupler changeover valve (27, 37) switches to the unlock-side position characterized in that when the changeover switch (28, 38) switches to the hold position, the boosting valve (25, 35) switches to the non-boosting position and the coupler changeover valve (27, 37) switches to the lock-side position. a coupler cylinder (21, 30) configured to be driven due to the supply of hydraulic fluid, in a locking direction to lock the attachment to the quick coupler and in an unlocking direction to unlock the attachment from the quick coupler; an actuator configured to be driven due to the supply of hydraulic fluid; a first hydraulic pump (23, 39) configured to be connected in parallel with the coupler cylinder (21, 31) and the actuator, and configured to supply the hydraulic fluid to the coupler cylinder (21, 31) and the actuator; a boosting valve (25, 35) configured to switch between a boosting position to raise a pressure of the hydraulic fluid supplied from the first hydraulic pump (23, 39) to the coupler cylinder (21, 31), and a non-boosting position not to change the pressure of the hydraulic fluid supplied from the first hydraulic pump (23, 39) to the coupler cylinder (21, 31); a coupler changeover valve (27, 37) configured to switch between a lock-side position to supply hydraulic fluid from the first hydraulic pump (23, 39) to the coupler cylinder (21, 31) so as to drive the coupler cylinder (21, 31) in the locking direction, and an unlock-side position to supply the hydraulic fluid from the first hydraulic pump (23, 39) to the coupler cylinder (21, 31) so as to drive the coupler cylinder (21, 31) in the unlocking direction; and a changeover switch (28, 38) configured to switch between an unlock position, a lock position, and a hold position, wherein when the changeover switch (28, 38) switches to the lock position, the boosting valve (25, 35) switches to the boosting position and the coupler changeover valve (27, 37) switches to the lock-side position; and when the changeover switch (28, 38) switches to the unlock position, the boosting valve (25, 35) switches to the boosting position and the coupler changeover valve (27, 37) switches to the unlock-side position characterized in that when the changeover switch (28, 38) switches to the hold position, the boosting valve (25, 35) switches to the non-boosting position and the coupler changeover valve (27, 37) switches to the lock-side position.