According to the present invention, a ladder having an adjustable length comprises: a first support including a first outer support and a first inner support inserted into the first outer support; a horizontal foothold for connecting the first supports to each other; a first gear formed at the inner side of the horizontal foothold and having a groove part formed therein; a second gear engaging with the first gear and changing the rotational direction of the first gear; a rotary bar coupled to the second gear; a third gear formed at the end of one side of the rotary bar; a screw bolt having a fourth gear, which engages with the third gear and is formed at the end thereof, formed inside the first inner support, formed in a stick type, and having threads formed on the outer side thereof; and a screw nut in which the screw bolt is formed, and to which the first inner support is coupled at the outer side thereof.