PURPOSE: To enable underwater photography with high image formation performance over a wide range from an infinite distance to a short distance corresponding to unmagnification by employing a system of two-group constitution of a front group which has positive refracting power and a rear group which has negative refracting power and extends the front group.

CONSTITUTION: A 1st lens group G1 which is the closest to an object side functions as a pressure-tight waterproof window. The 1st lens group G1 has no refracting power or extremely small refracting power. Then a 2nd lens group G2 and a 3rd lens group G3 have positive refracting power and a 4th lens group G4 has negative refracting power. For focusing from an infinite distance to a short distance, the 2nd lens group G2 and 3rd lens group G3 are moved on the optical axis to the object side away from the 4th lens group G4 while the group interval between the 2nd lens group G2 and 3rd lens group G3 is varied. Then various aberrations at the time of the focusing from the infinite distance to the short distance are excellently compensated by specifying the relation between the group intervals of those lens groups and a focusing coefficient.