An optical pickup 1 includes a collimator lens 13 for changing the shape of a laser light by moving on the optical axis AX from a base point. An objective lens 15 is set so as to form a spot of which size, when the collimator lens 13 is located at the base point, is minimum at a point apart from the light source side surface of an information recording medium 20 by a distance of L.sub.c defined by an expression (1): L.sub.c=(L.sub.0+L.sub.n)/2 (1), wherein L.sub.c is named as a thickness to the designed center; L.sub.0 is a distance between the light source side surface of the information recoding medium and an information recording face located nearest to the light source side surface; and L.sub.n is a distance between the light source side surface of the information recording medium and an information recording face located farthest from the light source side surface.