A hand-operated circular saw having blade cutting depth the adjustment device includes a hollow interior tubular body having a hand grasp portion, a head portion, a motor disposed in the head portion to operate a threaded axis protrude from a threaded circular protrusion on an outer side of head portion, an upper plate secured to the outer side including a circular hole engaged with the circular protrusions, a pair of pivotal tubes and a pair of horizontal arc protrusions on the top, a guard plate, a pad ring, an internally thread tube and a saw blade sequentially secured to the axis, a lower plate connected to a pair of cylinder rods inside the pivotal tubes, a pair of quickly operated bolts in the top of the pivotal tubes for adjusting the vertical positions of the lower plate and the saw blade, and a cutting depth adjustment device.