The present invention relates to an acyclic nucleoside phosphonate derivative represented by the following formula (1): in which represents single bond or double bond, R1, R2, R3, R7 and R8 are defined herein, Y represents —O—, —S—, —CH(Z)-, ═C(Z)-, —N(Z)-, ═N—, or —SiH(Z)-, wherein Z represents hydrogen, hydroxy or halogen, or represents C1-C7-alkyl, C1-C5-alkoxy, allyl, hydroxy-C1-C7-alkyl, C1-C7-aminoalkyl or phenyl, Q represents a group having the following formula: wherein X1, and X2 independently of one another represent hydrogen, amino, hydroxy or halogen, or represent C1-C7-alkyl, C1-C5-alkoxy, allyl, hydroxy-C1-C7-alkyl, phenyl or phenoxy each of which is optionally substituted by nitro or C1-C5-alkoxy, or represent C6-C10-arylthio which is optionally substituted by nitro, amino, C1-C6-alkyl or C1-C4-alkoxy, or represent C6-C12-arylamino, C1-C7-alkylamino, di(C1-C7-alkyl)amino, C3-C6-cycloalkylamino or a structure of wherein n denotes an integer of 1 or 2 and Y1 represents O, CH2 or N—R (R represents C1-C7-alkyl or C6-C12-aryl), which is useful as an antiviral agent (particularly, against hepatitis B virus), pharmaceutically acceptable salts, stereoisomers, and a process for the preparation thereof.